diff --git a/runtime/lib/object.cc b/runtime/lib/object.cc
index 6cb94ccc9f7..04afec92c9c 100644
--- a/runtime/lib/object.cc
+++ b/runtime/lib/object.cc
@@ -196,6 +196,19 @@ DEFINE_NATIVE_ENTRY(Type_equality, 0, 2) {
   return Bool::Get(type.IsEquivalent(other, TypeEquality::kSyntactical)).raw();
 }
 
+DEFINE_NATIVE_ENTRY(LibraryPrefix_isLoaded, 0, 1) {
+  const LibraryPrefix& prefix =
+      LibraryPrefix::CheckedHandle(zone, arguments->NativeArgAt(0));
+  return Bool::Get(prefix.is_loaded()).raw();
+}
+
+DEFINE_NATIVE_ENTRY(LibraryPrefix_setLoaded, 0, 1) {
+  const LibraryPrefix& prefix =
+      LibraryPrefix::CheckedHandle(zone, arguments->NativeArgAt(0));
+  prefix.set_is_loaded(true);
+  return Instance::null();
+}
+
 DEFINE_NATIVE_ENTRY(Internal_inquireIs64Bit, 0, 0) {
 #if defined(ARCH_IS_64_BIT)
   return Bool::True().raw();
diff --git a/runtime/vm/bootstrap_natives.h b/runtime/vm/bootstrap_natives.h
index 24608463bb0..6ce5668eb48 100644
--- a/runtime/vm/bootstrap_natives.h
+++ b/runtime/vm/bootstrap_natives.h
@@ -31,6 +31,8 @@ namespace dart {
   V(AbstractType_toString, 1)                                                  \
   V(Type_getHashCode, 1)                                                       \
   V(Type_equality, 2)                                                          \
+  V(LibraryPrefix_isLoaded, 1)                                                 \
+  V(LibraryPrefix_setLoaded, 1)                                                \
   V(Identical_comparison, 2)                                                   \
   V(Integer_bitAndFromInteger, 2)                                              \
   V(Integer_bitOrFromInteger, 2)                                               \
diff --git a/runtime/vm/clustered_snapshot.cc b/runtime/vm/clustered_snapshot.cc
index a999eb1f8db..25a8d76bf83 100644
--- a/runtime/vm/clustered_snapshot.cc
+++ b/runtime/vm/clustered_snapshot.cc
@@ -3248,6 +3248,7 @@ class LibraryPrefixDeserializationCluster : public DeserializationCluster {
       ReadFromTo(prefix);
       prefix->ptr()->num_imports_ = d->Read<uint16_t>();
       prefix->ptr()->is_deferred_load_ = d->Read<bool>();
+      prefix->ptr()->is_loaded_ = !prefix->ptr()->is_deferred_load_;
     }
   }
 };
diff --git a/runtime/vm/compiler/backend/il.cc b/runtime/vm/compiler/backend/il.cc
index 390b3aca49d..dc254d49f77 100644
--- a/runtime/vm/compiler/backend/il.cc
+++ b/runtime/vm/compiler/backend/il.cc
@@ -1097,7 +1097,11 @@ ConstantInstr::ConstantInstr(const Object& value, TokenPosition token_pos)
     // tables used for certain character classes are represented as TypedData,
     // and so those values are also neither immutable (as there are no immutable
     // TypedData values) or canonical.
-    ASSERT(value.IsTypeParameter() || value.IsArray() || value.IsTypedData());
+    //
+    // LibraryPrefixes are also never canonicalized since their equality is
+    // their identity.
+    ASSERT(value.IsTypeParameter() || value.IsArray() || value.IsTypedData() ||
+           value.IsLibraryPrefix());
   }
 #endif
 }
diff --git a/runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc b/runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc
index aba146c0dae..589ed9e8167 100644
--- a/runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc
+++ b/runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc
@@ -1363,9 +1363,9 @@ Fragment StreamingFlowGraphBuilder::BuildExpression(TokenPosition* position) {
     case kInstantiation:
       return BuildPartialTearoffInstantiation(position);
     case kLoadLibrary:
+      return BuildLibraryPrefixAction(position, Symbols::LoadLibrary());
     case kCheckLibraryIsLoaded:
-      ReadUInt();  // skip library index
-      return BuildFutureNullValue(position);
+      return BuildLibraryPrefixAction(position, Symbols::CheckLoaded());
     case kConstStaticInvocation:
     case kConstConstructorInvocation:
     case kConstListLiteral:
@@ -4068,6 +4068,26 @@ Fragment StreamingFlowGraphBuilder::BuildPartialTearoffInstantiation(
   return instructions;
 }
 
+Fragment StreamingFlowGraphBuilder::BuildLibraryPrefixAction(
+    TokenPosition* position,
+    const String& selector) {
+  const intptr_t dependency_index = ReadUInt();
+  const Library& current_library = Library::Handle(
+      Z, Class::Handle(Z, parsed_function()->function().origin()).library());
+  const Array& dependencies = Array::Handle(Z, current_library.dependencies());
+  const LibraryPrefix& prefix =
+      LibraryPrefix::CheckedZoneHandle(Z, dependencies.At(dependency_index));
+  const Function& function =
+      Function::ZoneHandle(Z, Library::Handle(Z, Library::CoreLibrary())
+                                  .LookupFunctionAllowPrivate(selector));
+  ASSERT(!function.IsNull());
+  Fragment instructions;
+  instructions += Constant(prefix);
+  instructions +=
+      StaticCall(TokenPosition::kNoSource, function, 1, ICData::kStatic);
+  return instructions;
+}
+
 Fragment StreamingFlowGraphBuilder::BuildExpressionStatement() {
   Fragment instructions = BuildExpression();  // read expression.
   instructions += Drop();
diff --git a/runtime/vm/compiler/frontend/kernel_binary_flowgraph.h b/runtime/vm/compiler/frontend/kernel_binary_flowgraph.h
index 2b1ba418ea6..c36a7c8232f 100644
--- a/runtime/vm/compiler/frontend/kernel_binary_flowgraph.h
+++ b/runtime/vm/compiler/frontend/kernel_binary_flowgraph.h
@@ -335,6 +335,8 @@ class StreamingFlowGraphBuilder : public KernelReaderHelper {
   Fragment BuildFutureNullValue(TokenPosition* position);
   Fragment BuildConstantExpression(TokenPosition* position, Tag tag);
   Fragment BuildPartialTearoffInstantiation(TokenPosition* position);
+  Fragment BuildLibraryPrefixAction(TokenPosition* position,
+                                    const String& selector);
 
   Fragment BuildExpressionStatement();
   Fragment BuildBlock();
diff --git a/runtime/vm/compiler/runtime_offsets_extracted.h b/runtime/vm/compiler/runtime_offsets_extracted.h
index 409fa7d7977..b7e6bda1f94 100644
--- a/runtime/vm/compiler/runtime_offsets_extracted.h
+++ b/runtime/vm/compiler/runtime_offsets_extracted.h
@@ -450,7 +450,7 @@ static constexpr dart::compiler::target::word Integer_InstanceSize = 4;
 static constexpr dart::compiler::target::word KernelProgramInfo_InstanceSize =
     64;
 static constexpr dart::compiler::target::word LanguageError_InstanceSize = 28;
-static constexpr dart::compiler::target::word Library_InstanceSize = 76;
+static constexpr dart::compiler::target::word Library_InstanceSize = 80;
 static constexpr dart::compiler::target::word LibraryPrefix_InstanceSize = 20;
 static constexpr dart::compiler::target::word LinkedHashMap_InstanceSize = 28;
 static constexpr dart::compiler::target::word LocalVarDescriptors_InstanceSize =
@@ -944,7 +944,7 @@ static constexpr dart::compiler::target::word Integer_InstanceSize = 8;
 static constexpr dart::compiler::target::word KernelProgramInfo_InstanceSize =
     128;
 static constexpr dart::compiler::target::word LanguageError_InstanceSize = 48;
-static constexpr dart::compiler::target::word Library_InstanceSize = 144;
+static constexpr dart::compiler::target::word Library_InstanceSize = 152;
 static constexpr dart::compiler::target::word LibraryPrefix_InstanceSize = 40;
 static constexpr dart::compiler::target::word LinkedHashMap_InstanceSize = 56;
 static constexpr dart::compiler::target::word LocalVarDescriptors_InstanceSize =
@@ -1429,7 +1429,7 @@ static constexpr dart::compiler::target::word Integer_InstanceSize = 4;
 static constexpr dart::compiler::target::word KernelProgramInfo_InstanceSize =
     64;
 static constexpr dart::compiler::target::word LanguageError_InstanceSize = 28;
-static constexpr dart::compiler::target::word Library_InstanceSize = 76;
+static constexpr dart::compiler::target::word Library_InstanceSize = 80;
 static constexpr dart::compiler::target::word LibraryPrefix_InstanceSize = 20;
 static constexpr dart::compiler::target::word LinkedHashMap_InstanceSize = 28;
 static constexpr dart::compiler::target::word LocalVarDescriptors_InstanceSize =
@@ -1924,7 +1924,7 @@ static constexpr dart::compiler::target::word Integer_InstanceSize = 8;
 static constexpr dart::compiler::target::word KernelProgramInfo_InstanceSize =
     128;
 static constexpr dart::compiler::target::word LanguageError_InstanceSize = 48;
-static constexpr dart::compiler::target::word Library_InstanceSize = 144;
+static constexpr dart::compiler::target::word Library_InstanceSize = 152;
 static constexpr dart::compiler::target::word LibraryPrefix_InstanceSize = 40;
 static constexpr dart::compiler::target::word LinkedHashMap_InstanceSize = 56;
 static constexpr dart::compiler::target::word LocalVarDescriptors_InstanceSize =
@@ -2408,7 +2408,7 @@ static constexpr dart::compiler::target::word Integer_InstanceSize = 4;
 static constexpr dart::compiler::target::word KernelProgramInfo_InstanceSize =
     64;
 static constexpr dart::compiler::target::word LanguageError_InstanceSize = 28;
-static constexpr dart::compiler::target::word Library_InstanceSize = 76;
+static constexpr dart::compiler::target::word Library_InstanceSize = 80;
 static constexpr dart::compiler::target::word LibraryPrefix_InstanceSize = 20;
 static constexpr dart::compiler::target::word LinkedHashMap_InstanceSize = 28;
 static constexpr dart::compiler::target::word LocalVarDescriptors_InstanceSize =
@@ -2896,7 +2896,7 @@ static constexpr dart::compiler::target::word Integer_InstanceSize = 8;
 static constexpr dart::compiler::target::word KernelProgramInfo_InstanceSize =
     128;
 static constexpr dart::compiler::target::word LanguageError_InstanceSize = 48;
-static constexpr dart::compiler::target::word Library_InstanceSize = 144;
+static constexpr dart::compiler::target::word Library_InstanceSize = 152;
 static constexpr dart::compiler::target::word LibraryPrefix_InstanceSize = 40;
 static constexpr dart::compiler::target::word LinkedHashMap_InstanceSize = 56;
 static constexpr dart::compiler::target::word LocalVarDescriptors_InstanceSize =
@@ -3375,7 +3375,7 @@ static constexpr dart::compiler::target::word Integer_InstanceSize = 4;
 static constexpr dart::compiler::target::word KernelProgramInfo_InstanceSize =
     64;
 static constexpr dart::compiler::target::word LanguageError_InstanceSize = 28;
-static constexpr dart::compiler::target::word Library_InstanceSize = 76;
+static constexpr dart::compiler::target::word Library_InstanceSize = 80;
 static constexpr dart::compiler::target::word LibraryPrefix_InstanceSize = 20;
 static constexpr dart::compiler::target::word LinkedHashMap_InstanceSize = 28;
 static constexpr dart::compiler::target::word LocalVarDescriptors_InstanceSize =
@@ -3864,7 +3864,7 @@ static constexpr dart::compiler::target::word Integer_InstanceSize = 8;
 static constexpr dart::compiler::target::word KernelProgramInfo_InstanceSize =
     128;
 static constexpr dart::compiler::target::word LanguageError_InstanceSize = 48;
-static constexpr dart::compiler::target::word Library_InstanceSize = 144;
+static constexpr dart::compiler::target::word Library_InstanceSize = 152;
 static constexpr dart::compiler::target::word LibraryPrefix_InstanceSize = 40;
 static constexpr dart::compiler::target::word LinkedHashMap_InstanceSize = 56;
 static constexpr dart::compiler::target::word LocalVarDescriptors_InstanceSize =
@@ -4384,7 +4384,7 @@ static constexpr dart::compiler::target::word
     AOT_KernelProgramInfo_InstanceSize = 64;
 static constexpr dart::compiler::target::word AOT_LanguageError_InstanceSize =
     28;
-static constexpr dart::compiler::target::word AOT_Library_InstanceSize = 72;
+static constexpr dart::compiler::target::word AOT_Library_InstanceSize = 76;
 static constexpr dart::compiler::target::word AOT_LibraryPrefix_InstanceSize =
     20;
 static constexpr dart::compiler::target::word AOT_LinkedHashMap_InstanceSize =
@@ -4919,7 +4919,7 @@ static constexpr dart::compiler::target::word
     AOT_KernelProgramInfo_InstanceSize = 128;
 static constexpr dart::compiler::target::word AOT_LanguageError_InstanceSize =
     48;
-static constexpr dart::compiler::target::word AOT_Library_InstanceSize = 136;
+static constexpr dart::compiler::target::word AOT_Library_InstanceSize = 144;
 static constexpr dart::compiler::target::word AOT_LibraryPrefix_InstanceSize =
     40;
 static constexpr dart::compiler::target::word AOT_LinkedHashMap_InstanceSize =
@@ -5458,7 +5458,7 @@ static constexpr dart::compiler::target::word
     AOT_KernelProgramInfo_InstanceSize = 128;
 static constexpr dart::compiler::target::word AOT_LanguageError_InstanceSize =
     48;
-static constexpr dart::compiler::target::word AOT_Library_InstanceSize = 136;
+static constexpr dart::compiler::target::word AOT_Library_InstanceSize = 144;
 static constexpr dart::compiler::target::word AOT_LibraryPrefix_InstanceSize =
     40;
 static constexpr dart::compiler::target::word AOT_LinkedHashMap_InstanceSize =
@@ -5984,7 +5984,7 @@ static constexpr dart::compiler::target::word
     AOT_KernelProgramInfo_InstanceSize = 64;
 static constexpr dart::compiler::target::word AOT_LanguageError_InstanceSize =
     28;
-static constexpr dart::compiler::target::word AOT_Library_InstanceSize = 72;
+static constexpr dart::compiler::target::word AOT_Library_InstanceSize = 76;
 static constexpr dart::compiler::target::word AOT_LibraryPrefix_InstanceSize =
     20;
 static constexpr dart::compiler::target::word AOT_LinkedHashMap_InstanceSize =
@@ -6512,7 +6512,7 @@ static constexpr dart::compiler::target::word
     AOT_KernelProgramInfo_InstanceSize = 128;
 static constexpr dart::compiler::target::word AOT_LanguageError_InstanceSize =
     48;
-static constexpr dart::compiler::target::word AOT_Library_InstanceSize = 136;
+static constexpr dart::compiler::target::word AOT_Library_InstanceSize = 144;
 static constexpr dart::compiler::target::word AOT_LibraryPrefix_InstanceSize =
     40;
 static constexpr dart::compiler::target::word AOT_LinkedHashMap_InstanceSize =
@@ -7044,7 +7044,7 @@ static constexpr dart::compiler::target::word
     AOT_KernelProgramInfo_InstanceSize = 128;
 static constexpr dart::compiler::target::word AOT_LanguageError_InstanceSize =
     48;
-static constexpr dart::compiler::target::word AOT_Library_InstanceSize = 136;
+static constexpr dart::compiler::target::word AOT_Library_InstanceSize = 144;
 static constexpr dart::compiler::target::word AOT_LibraryPrefix_InstanceSize =
     40;
 static constexpr dart::compiler::target::word AOT_LinkedHashMap_InstanceSize =
diff --git a/runtime/vm/isolate_reload.cc b/runtime/vm/isolate_reload.cc
index 74556354e9d..6c83404d48a 100644
--- a/runtime/vm/isolate_reload.cc
+++ b/runtime/vm/isolate_reload.cc
@@ -738,7 +738,7 @@ bool IsolateGroupReloadContext::Reload(bool force_reload,
 
   // Ensure all functions on the stack have unoptimized code.
   // Deoptimize all code that had optimizing decisions that are dependent on
-  // assumptions from field guards or CHA.
+  // assumptions from field guards or CHA or deferred library prefixes.
   // TODO(johnmccutchan): Deoptimizing dependent code here (before the reload)
   // is paranoid. This likely can be moved to the commit phase.
   ForEachIsolate([&](Isolate* isolate) {
@@ -1312,6 +1312,8 @@ void IsolateReloadContext::DeoptimizeDependentCode() {
   }
 
   DeoptimizeTypeTestingStubs();
+
+  // TODO(rmacnak): Also call LibraryPrefix::InvalidateDependentCode.
 }
 
 void IsolateGroupReloadContext::CheckpointSharedClassTable() {
diff --git a/runtime/vm/kernel_loader.cc b/runtime/vm/kernel_loader.cc
index 7bfd7d76db9..1d0d5437726 100644
--- a/runtime/vm/kernel_loader.cc
+++ b/runtime/vm/kernel_loader.cc
@@ -1305,6 +1305,7 @@ void KernelLoader::LoadLibraryImportsAndExports(Library* library,
   LibraryPrefix& library_prefix = LibraryPrefix::Handle(Z);
 
   const intptr_t deps_count = helper_.ReadListLength();
+  const Array& deps = Array::Handle(Array::New(deps_count));
   for (intptr_t dep = 0; dep < deps_count; ++dep) {
     LibraryDependencyHelper dependency_helper(&helper_);
 
@@ -1388,12 +1389,21 @@ void KernelLoader::LoadLibraryImportsAndExports(Library* library,
         }
       }
     }
+
     if (FLAG_enable_mirrors && dependency_helper.annotation_count_ > 0) {
       ASSERT(annotations_kernel_offset > 0);
       ns.AddMetadata(toplevel_class, TokenPosition::kNoSource,
                      annotations_kernel_offset);
     }
+
+    if (prefix.IsNull()) {
+      deps.SetAt(dep, ns);
+    } else {
+      deps.SetAt(dep, library_prefix);
+    }
   }
+
+  library->set_dependencies(deps);
 }
 
 void KernelLoader::LoadPreliminaryClass(ClassHelper* class_helper,
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index cefce52562d..04f500e9eda 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -11081,27 +11081,6 @@ void ClassDictionaryIterator::MoveToNextClass() {
   }
 }
 
-LibraryPrefixIterator::LibraryPrefixIterator(const Library& library)
-    : DictionaryIterator(library) {
-  Advance();
-}
-
-LibraryPrefixPtr LibraryPrefixIterator::GetNext() {
-  ASSERT(HasNext());
-  int ix = next_ix_++;
-  Object& obj = Object::Handle(array_.At(ix));
-  Advance();
-  return LibraryPrefix::Cast(obj).raw();
-}
-
-void LibraryPrefixIterator::Advance() {
-  Object& obj = Object::Handle(array_.At(next_ix_));
-  while (!obj.IsLibraryPrefix() && HasNext()) {
-    next_ix_++;
-    obj = array_.At(next_ix_);
-  }
-}
-
 static void ReportTooManyImports(const Library& lib) {
   const String& url = String::Handle(lib.url());
   Report::MessageF(Report::kError, Script::Handle(lib.LookupScript(url)),
@@ -12075,6 +12054,10 @@ void Library::set_toplevel_class(const Class& value) const {
   StorePointer(&raw_ptr()->toplevel_class_, value.raw());
 }
 
+void Library::set_dependencies(const Array& deps) const {
+  StorePointer(&raw_ptr()->dependencies_, deps.raw());
+}
+
 void Library::set_metadata(const GrowableObjectArray& value) const {
   StorePointer(&raw_ptr()->metadata_, value.raw());
 }
@@ -12943,6 +12926,7 @@ LibraryPrefixPtr LibraryPrefix::New(const String& name,
   result.set_num_imports(0);
   result.set_importer(importer);
   result.StoreNonPointer(&result.raw_ptr()->is_deferred_load_, deferred_load);
+  result.StoreNonPointer(&result.raw_ptr()->is_loaded_, !deferred_load);
   result.set_imports(Array::Handle(Array::New(kInitialSize)));
   result.AddImport(import);
   return result.raw();
@@ -12970,8 +12954,7 @@ void LibraryPrefix::set_importer(const Library& value) const {
 
 const char* LibraryPrefix::ToCString() const {
   const String& prefix = String::Handle(name());
-  return OS::SCreate(Thread::Current()->zone(), "LibraryPrefix:'%s'",
-                     prefix.ToCString());
+  return prefix.ToCString();
 }
 
 void Namespace::set_metadata_field(const Field& value) const {
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 4e476209d58..bd5baa1a0f5 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -4543,7 +4543,6 @@ class DictionaryIterator : public ValueObject {
   int next_ix_;     // Index of next element.
 
   friend class ClassDictionaryIterator;
-  friend class LibraryPrefixIterator;
   DISALLOW_COPY_AND_ASSIGN(DictionaryIterator);
 };
 
@@ -4574,16 +4573,6 @@ class ClassDictionaryIterator : public DictionaryIterator {
   DISALLOW_COPY_AND_ASSIGN(ClassDictionaryIterator);
 };
 
-class LibraryPrefixIterator : public DictionaryIterator {
- public:
-  explicit LibraryPrefixIterator(const Library& library);
-  LibraryPrefixPtr GetNext();
-
- private:
-  void Advance();
-  DISALLOW_COPY_AND_ASSIGN(LibraryPrefixIterator);
-};
-
 class Library : public Object {
  public:
   StringPtr name() const { return raw_ptr()->name_; }
@@ -4738,6 +4727,9 @@ class Library : public Object {
   NamespacePtr ImportAt(intptr_t index) const;
   LibraryPtr ImportLibraryAt(intptr_t index) const;
 
+  ArrayPtr dependencies() const { return raw_ptr()->dependencies_; }
+  void set_dependencies(const Array& deps) const;
+
   void DropDependenciesAndCaches() const;
 
   // Resolving native methods for script loaded in the library.
@@ -7342,6 +7334,10 @@ class LibraryPrefix : public Instance {
   void AddImport(const Namespace& import) const;
 
   bool is_deferred_load() const { return raw_ptr()->is_deferred_load_; }
+  bool is_loaded() const { return raw_ptr()->is_loaded_; }
+  void set_is_loaded(bool value) const {
+    return StoreNonPointer(&raw_ptr()->is_loaded_, value);
+  }
 
   static intptr_t InstanceSize() {
     return RoundedAllocationSize(sizeof(LibraryPrefixLayout));
diff --git a/runtime/vm/object_reload.cc b/runtime/vm/object_reload.cc
index 65677896917..cc6f720fcb4 100644
--- a/runtime/vm/object_reload.cc
+++ b/runtime/vm/object_reload.cc
@@ -692,6 +692,29 @@ class InstanceSizeConflict : public ClassReasonForCancelling {
   }
 };
 
+class UnimplementedDeferredLibrary : public ReasonForCancelling {
+ public:
+  UnimplementedDeferredLibrary(Zone* zone,
+                               const Library& from,
+                               const Library& to,
+                               const String& name)
+      : ReasonForCancelling(zone), from_(from), to_(to), name_(name) {}
+
+ private:
+  const Library& from_;
+  const Library& to_;
+  const String& name_;
+
+  StringPtr ToString() {
+    const String& lib_url = String::Handle(to_.url());
+    from_.ToCString();
+    return String::NewFormatted(
+        "Reloading support for deferred loading has not yet been implemented:"
+        " library '%s' has deferred import '%s'",
+        lib_url.ToCString(), name_.ToCString());
+  }
+};
+
 // This is executed before iterating over the instances.
 void Class::CheckReload(const Class& replacement,
                         IsolateReloadContext* context) const {
@@ -889,7 +912,23 @@ bool Class::CanReloadPreFinalized(const Class& replacement,
 
 void Library::CheckReload(const Library& replacement,
                           IsolateReloadContext* context) const {
-  // Currently no library properties will prevent a reload.
+  // TODO(26878): If the replacement library uses deferred loading,
+  // reject it.  We do not yet support reloading deferred libraries.
+  Object& object = Object::Handle();
+  LibraryPrefix& prefix = LibraryPrefix::Handle();
+  DictionaryIterator it(replacement);
+  while (it.HasNext()) {
+    object = it.GetNext();
+    if (!object.IsLibraryPrefix()) continue;
+    prefix ^= object.raw();
+    if (prefix.is_deferred_load()) {
+      const String& prefix_name = String::Handle(prefix.name());
+      context->group_reload_context()->AddReasonForCancelling(
+          new (context->zone()) UnimplementedDeferredLibrary(
+              context->zone(), *this, replacement, prefix_name));
+      return;
+    }
+  }
 }
 
 void CallSiteResetter::Reset(const ICData& ic) {
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index ee472ace028..18702888c7f 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -1305,6 +1305,7 @@ class LibraryLayout : public ObjectLayout {
   GrowableObjectArrayPtr used_scripts_;
   ArrayPtr imports_;  // List of Namespaces imported without prefix.
   ArrayPtr exports_;  // List of re-exported Namespaces.
+  ArrayPtr dependencies_;
   ExternalTypedDataPtr kernel_data_;
   ObjectPtr* to_snapshot(Snapshot::Kind kind) {
     switch (kind) {
@@ -2130,6 +2131,7 @@ class LibraryPrefixLayout : public InstanceLayout {
   }
   uint16_t num_imports_;  // Number of library entries in libraries_.
   bool is_deferred_load_;
+  bool is_loaded_;
 };
 
 class TypeArgumentsLayout : public InstanceLayout {
diff --git a/runtime/vm/symbols.h b/runtime/vm/symbols.h
index a5249ed9fb2..fb24834f495 100644
--- a/runtime/vm/symbols.h
+++ b/runtime/vm/symbols.h
@@ -44,6 +44,7 @@ class ObjectPointerVisitor;
   V(Call, "call")                                                              \
   V(Cancel, "cancel")                                                          \
   V(CastError, "_CastError")                                                   \
+  V(CheckLoaded, "_checkLoaded")                                               \
   V(Class, "Class")                                                            \
   V(ClassID, "ClassID")                                                        \
   V(ClearAsyncThreadStackTrace, "_clearAsyncThreadStackTrace")                 \
@@ -206,6 +207,7 @@ class ObjectPointerVisitor;
   V(ListFactory, "List.")                                                      \
   V(ListFilledFactory, "List.filled")                                          \
   V(ListLiteralFactory, "List._fromLiteral")                                   \
+  V(LoadLibrary, "_loadLibrary")                                               \
   V(LocalVarDescriptors, "LocalVarDescriptors")                                \
   V(Map, "Map")                                                                \
   V(MapLiteralFactory, "Map._fromLiteral")                                     \
diff --git a/sdk/lib/_internal/vm/lib/lib_prefix.dart b/sdk/lib/_internal/vm/lib/lib_prefix.dart
index 00b934c905f..5135a26926f 100644
--- a/sdk/lib/_internal/vm/lib/lib_prefix.dart
+++ b/sdk/lib/_internal/vm/lib/lib_prefix.dart
@@ -13,7 +13,32 @@ class _LibraryPrefix {
     throw "Unreachable";
   }
 
-  bool isLoaded() => true;
-
-  loadLibrary() => new Future.value(true);
+  bool _isLoaded() native "LibraryPrefix_isLoaded";
+  void _setLoaded() native "LibraryPrefix_setLoaded";
+}
+
+class _DeferredNotLoadedError extends Error implements NoSuchMethodError {
+  final _LibraryPrefix prefix;
+
+  _DeferredNotLoadedError(this.prefix);
+
+  String toString() {
+    return "Deferred library $prefix was not loaded.";
+  }
+}
+
+@pragma("vm:entry-point")
+@pragma("vm:never-inline") // Don't duplicate prefix checking code.
+Future<void> _loadLibrary(_LibraryPrefix prefix) {
+  return new Future<void>(() {
+    prefix._setLoaded();
+  });
+}
+
+@pragma("vm:entry-point")
+@pragma("vm:never-inline") // Don't duplicate prefix checking code.
+void _checkLoaded(_LibraryPrefix prefix) {
+  if (!prefix._isLoaded()) {
+    throw new _DeferredNotLoadedError(prefix);
+  }
 }
diff --git a/sdk_nnbd/lib/_internal/vm/lib/lib_prefix.dart b/sdk_nnbd/lib/_internal/vm/lib/lib_prefix.dart
index f588ee33e4f..d8e52fdb3e0 100644
--- a/sdk_nnbd/lib/_internal/vm/lib/lib_prefix.dart
+++ b/sdk_nnbd/lib/_internal/vm/lib/lib_prefix.dart
@@ -11,7 +11,32 @@ class _LibraryPrefix {
     throw "Unreachable";
   }
 
-  bool isLoaded() => true;
-
-  loadLibrary() => new Future.value(true);
+  bool _isLoaded() native "LibraryPrefix_isLoaded";
+  void _setLoaded() native "LibraryPrefix_setLoaded";
+}
+
+class _DeferredNotLoadedError extends Error implements NoSuchMethodError {
+  final _LibraryPrefix prefix;
+
+  _DeferredNotLoadedError(this.prefix);
+
+  String toString() {
+    return "Deferred library $prefix was not loaded.";
+  }
+}
+
+@pragma("vm:entry-point")
+@pragma("vm:never-inline") // Don't duplicate prefix checking code.
+Future<void> _loadLibrary(_LibraryPrefix prefix) {
+  return new Future<void>(() {
+    prefix._setLoaded();
+  });
+}
+
+@pragma("vm:entry-point")
+@pragma("vm:never-inline") // Don't duplicate prefix checking code.
+void _checkLoaded(_LibraryPrefix prefix) {
+  if (!prefix._isLoaded()) {
+    throw new _DeferredNotLoadedError(prefix);
+  }
 }