diff --git a/tests/language/closure/param_null_to_object_test.dart b/tests/language/closure/param_null_to_object_test.dart
index e34f400670d..b1139f3a9c1 100644
--- a/tests/language/closure/param_null_to_object_test.dart
+++ b/tests/language/closure/param_null_to_object_test.dart
@@ -28,4 +28,39 @@ main() {
   Expect.isTrue(h is int Function(String));
   Expect.isFalse(h is int Function(Object));
   Expect.isFalse(h is int Function(Object?));
+
+  int Function(Null) i = (Null x) => 1; // Runtime type is int Function(Null)
+  Expect.isTrue(i is int Function(Null));
+  Expect.isTrue(i is int Function(Never));
+  Expect.isFalse(i is int Function(String));
+  Expect.isFalse(i is int Function(Object));
+  Expect.isFalse(i is int Function(Object?));
+
+  int Function(Never) j = (Never x) => 1; // Runtime type is int Function(Never)
+  Expect.isFalse(j is int Function(Null));
+  Expect.isTrue(j is int Function(Never));
+  Expect.isFalse(j is int Function(String));
+  Expect.isFalse(j is int Function(Object));
+  Expect.isFalse(j is int Function(Object?));
+
+  // Test that the criteria used for weakening the parameter type
+  // is that the downwards context parameter type is a subtype of Null
+  void test<X extends Null, Y extends Never>() {
+    int Function(X) f = (x) => 1; // Runtime type is int Function(Object?)
+    Expect.isTrue(f is int Function(Null));
+    Expect.isTrue(f is int Function(Never));
+    Expect.isTrue(f is int Function(String));
+    Expect.isTrue(f is int Function(Object));
+    Expect.isTrue(f is int Function(Object?));
+
+    int Function(Y) g = (x) => 1; // Runtime type is int Function(Object?)
+    Expect.isTrue(g is int Function(Null));
+    Expect.isTrue(g is int Function(Never));
+    Expect.isTrue(g is int Function(String));
+    Expect.isTrue(g is int Function(Object));
+    Expect.isTrue(g is int Function(Object?));
+  }
+
+  test<Null, Never>();
+  test<Never, Never>();
 }
diff --git a/tests/language/nnbd/const/potentially_constant_types_error_test.dart b/tests/language/nnbd/const/potentially_constant_types_error_test.dart
new file mode 100644
index 00000000000..02428360659
--- /dev/null
+++ b/tests/language/nnbd/const/potentially_constant_types_error_test.dart
@@ -0,0 +1,45 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Requirements=nnbd-strong
+
+/// Test instance checks and casts in constants may use potentially constant
+/// types, and cause compile time errors when the casts fail.
+
+class C1<T> {
+  final t;
+
+  /// Check casts to T
+  const C1.test(dynamic x) : t = x as T;
+}
+
+class C2<T> {
+  final l;
+
+  /// Check casts to List<T>
+  const C2.test(dynamic x) : l = x as List<T>;
+}
+
+void main() {
+  const c1 = C1<int>.test("hello");
+  //         ^
+  // [analyzer] unspecified
+  // [cfe] Constant evaluation error:
+  const c2 = C1<int>.test(null);
+  //         ^
+  // [analyzer] unspecified
+  // [cfe] unspecified
+  const c3 = C2<int>.test(<num>[0]);
+  //         ^
+  // [analyzer] unspecified
+  // [cfe] Constant evaluation error:
+  const c4 = C2<int>.test("hello");
+  //         ^
+  // [analyzer] unspecified
+  // [cfe] Constant evaluation error:
+  const c5 = C2<int>.test(null);
+  //         ^
+  // [analyzer] unspecified
+  // [cfe] unspecified
+}
diff --git a/tests/language/nnbd/const/potentially_constant_types_test.dart b/tests/language/nnbd/const/potentially_constant_types_test.dart
new file mode 100644
index 00000000000..7e45aeb0724
--- /dev/null
+++ b/tests/language/nnbd/const/potentially_constant_types_test.dart
@@ -0,0 +1,143 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Requirements=nnbd-strong
+
+/// Test instance checks and casts in constants may use potentially constant
+/// types, and evaluate appropriately.
+
+import 'package:expect/expect.dart';
+
+class C<T> {
+  final bool isT;
+  final bool isListT;
+  final T? t;
+  final List<T>? l;
+
+  /// Check instance tests in isolation
+  const C.test1(dynamic x)
+      : isT = x is T,
+        isListT = x is List<T>,
+        t = null,
+        l = null;
+
+  /// Check casts to T in isolation
+  const C.test2(dynamic x)
+      : isT = true,
+        isListT = false,
+        t = x as T,
+        l = null;
+
+  /// Check casts to List<T> in isolation
+  const C.test3(dynamic x)
+      : isT = false,
+        isListT = true,
+        t = null,
+        l = x as List<T>;
+
+  /// Combine instance checks with casts, conditional expressions, promotion
+  const C.test4(dynamic x)
+      : isT = x is T,
+        isListT = x is List<T>,
+        t = (x is T) ? x : null,
+        l = (x is List<T>) ? x : null;
+}
+
+void main() {
+  {
+    // Test instance checks of T
+    const c1 = C<int>.test1(0);
+    const c2 = C<int>.test1(0);
+    const c3 = C<int>.test1(1);
+    const c4 = C<int>.test1("hello");
+    const c5 = C<int>.test1(null);
+    Expect.identical(c1, c2);
+    Expect.identical(c1, c3);
+    Expect.notIdentical(c1, c4);
+    Expect.notIdentical(c1, c5);
+    Expect.isTrue(c1.isT);
+    Expect.isTrue(c2.isT);
+    Expect.isTrue(c3.isT);
+    Expect.isFalse(c4.isT);
+    Expect.isFalse(c5.isT);
+    Expect.isFalse(c1.isListT);
+    Expect.isFalse(c2.isListT);
+    Expect.isFalse(c3.isListT);
+    Expect.isFalse(c4.isListT);
+    Expect.isFalse(c5.isListT);
+  }
+  {
+    // Test instance checks of List<T>
+    const c1 = C<int>.test1(<int>[0]);
+    const c2 = C<int>.test1(<int>[0]);
+    const c3 = C<int>.test1(<int>[1]);
+    const c4 = C<int>.test1(<num>[1]);
+    const c5 = C<num>.test1(<int>[1]);
+    Expect.identical(c1, c2);
+    Expect.identical(c1, c3);
+    Expect.notIdentical(c1, c4);
+    Expect.notIdentical(c1, c5);
+    Expect.notIdentical(c4, c5);
+    Expect.isFalse(c1.isT);
+    Expect.isFalse(c2.isT);
+    Expect.isFalse(c3.isT);
+    Expect.isFalse(c4.isT);
+    Expect.isFalse(c5.isT);
+    Expect.isTrue(c1.isListT);
+    Expect.isTrue(c2.isListT);
+    Expect.isTrue(c3.isListT);
+    Expect.isFalse(c4.isListT);
+    Expect.isTrue(c5.isListT);
+  }
+  {
+    // Test casts to T
+    const c1 = C<int>.test2(0);
+    const c2 = C<int>.test2(0);
+    const c3 = C<num>.test2(1);
+    Expect.identical(c1, c2);
+    Expect.notIdentical(c1, c3);
+  }
+  {
+    // Test casts to List<T>
+    const c1 = C<int>.test3(<int>[0]);
+    const c2 = C<int>.test3(<int>[0]);
+    const c3 = C<num>.test3(<int>[0]);
+    Expect.identical(c1, c2);
+    Expect.notIdentical(c1, c3);
+  }
+
+  {
+    // Combined tests
+    const c1 = C<num>.test4(0);
+    const c2 = C<num>.test4("hello");
+    const c3 = C<num>.test4(<int>[0]);
+    const c4 = C<num>.test4(<String>["hello"]);
+    const c5 = C<int>.test4(<num>[0]);
+
+    Expect.isTrue(c1.isT);
+    Expect.isFalse(c1.isListT);
+    Expect.equals(c1.t, 0);
+    Expect.equals(c1.l, null);
+
+    Expect.isFalse(c2.isT);
+    Expect.isFalse(c2.isListT);
+    Expect.equals(c2.t, null);
+    Expect.equals(c2.l, null);
+
+    Expect.isFalse(c3.isT);
+    Expect.isTrue(c3.isListT);
+    Expect.equals(c3.t, null);
+    Expect.identical(c3.l, const <int>[0]);
+
+    Expect.isFalse(c4.isT);
+    Expect.isFalse(c4.isListT);
+    Expect.equals(c4.t, null);
+    Expect.equals(c4.l, null);
+
+    Expect.isFalse(c5.isT);
+    Expect.isFalse(c5.isListT);
+    Expect.equals(c5.t, null);
+    Expect.equals(c5.l, null);
+  }
+}
diff --git a/tests/language/nnbd/never/never_error_test.dart b/tests/language/nnbd/never/never_error_test.dart
new file mode 100644
index 00000000000..9ce76097145
--- /dev/null
+++ b/tests/language/nnbd/never/never_error_test.dart
@@ -0,0 +1,126 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Requirements=nnbd-strong
+
+/// Test that `Never` has all members, and that as a result no extensions
+/// apply to it.  Also test that `Never` is inferred as the type of a
+/// throw expression, and as the return type of function literals that
+/// never return.
+
+import 'package:expect/expect.dart';
+
+/// A call of the form staticErrorIfNotNever(t) will produce a static error
+/// unless `t` has type `Never`.
+void staticErrorIfNotNever<T extends Never>(T t) {}
+
+void neverHasAllMembers(Never x) {
+  {
+    // getters
+    var t = x.arglebargle;
+    staticErrorIfNotNever(t);
+  }
+  {
+    // setter
+    x.arglebargle = 3;
+  }
+  {
+    // operator[]
+    x[0] = 3;
+  }
+  {
+    // methods
+    var t = x.arglebargle(0);
+    staticErrorIfNotNever(t);
+  }
+  {
+    // methods with named parameters
+    var t = x.arglebargle(foo: 0);
+    staticErrorIfNotNever(t);
+  }
+  {
+    // call method
+    var t = x(3);
+    staticErrorIfNotNever(t);
+  }
+  {
+    // Object members
+    staticErrorIfNotNever(x.toString());
+    staticErrorIfNotNever(x.toString);
+    staticErrorIfNotNever(x.runtimeType);
+    staticErrorIfNotNever(x.noSuchMethod);
+    staticErrorIfNotNever(x.noSuchMethod(x));
+    staticErrorIfNotNever(x.hashCode);
+    staticErrorIfNotNever(x == x);
+    staticErrorIfNotNever(x == 3);
+    staticErrorIfNotNever(3 == x);
+//  ^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.COULD_NOT_INFER
+// [cfe] Inferred type argument 'bool' doesn't conform to the bound 'Never' of the type variable 'T' on 'staticErrorIfNotNever'.
+  }
+}
+
+extension NeverExt on Never {
+  int neverMethod() => 3;
+}
+
+extension ObjectExt on Object {
+  int objectMethod() => 3;
+}
+
+void extensionsDontApply(Never x) {
+  {
+    var t = x.neverMethod();
+    staticErrorIfNotNever(t);
+  }
+  {
+    var t = x.objectMethod();
+    staticErrorIfNotNever(t);
+  }
+  {
+    var t = NeverExt(x).neverMethod();
+    staticErrorIfNotNever(t);
+//  ^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.COULD_NOT_INFER
+// [cfe] Inferred type argument 'int' doesn't conform to the bound 'Never' of the type variable 'T' on 'staticErrorIfNotNever'.
+  }
+  {
+    var t = ObjectExt(x).objectMethod();
+    staticErrorIfNotNever(t);
+//  ^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.COULD_NOT_INFER
+// [cfe] Inferred type argument 'int' doesn't conform to the bound 'Never' of the type variable 'T' on 'staticErrorIfNotNever'.
+  }
+}
+
+void throwHasTypeNever() {
+  var t = throw "hello";
+  staticErrorIfNotNever(t);
+}
+
+void neverReturns(bool b) {
+  {
+    var f = () => throw "Unreachable";
+    var t = f();
+    staticErrorIfNotNever(t);
+  }
+  {
+    var f = () {
+      if (b) {
+        throw "argle";
+      } else {
+        throw "bargle";
+      }
+    };
+    var t = f();
+    staticErrorIfNotNever(t);
+  }
+}
+
+void main() {
+  neverHasAllMembers(throw "Unreachable");
+  extensionsDontApply(throw "Unreachable");
+  throwHasTypeNever();
+  neverReturns(true);
+}
diff --git a/tests/language/nnbd/static_errors/local_function_inference.dart b/tests/language/nnbd/static_errors/local_function_inference.dart
new file mode 100644
index 00000000000..9cb6a37b150
--- /dev/null
+++ b/tests/language/nnbd/static_errors/local_function_inference.dart
@@ -0,0 +1,29 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// Test that the type of a local function is an error if local function type
+/// inference requires the type of the function being inferred.
+
+void main() {
+  f() {
+    return 3;
+  }
+
+  f().arglebargle;
+  //  ^^^^^^^^^^^
+  // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_GETTER
+  // [cfe] The getter 'arglebargle' isn't defined for the class 'int'.
+  f().isEven; // Inferred type is int
+
+  g() {
+    if (f() == 3) {
+      return g();
+    } else {
+      return 3;
+    }
+  }
+  //  ^
+  // [analyzer] undefined
+  // [cfe] undefined
+}
diff --git a/tests/language/nnbd/type_promotion/downwards_inference_error_test.dart b/tests/language/nnbd/type_promotion/downwards_inference_error_test.dart
new file mode 100644
index 00000000000..d54b522ccf2
--- /dev/null
+++ b/tests/language/nnbd/type_promotion/downwards_inference_error_test.dart
@@ -0,0 +1,152 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// Test that downwards inference uses the promoted type of the variable in an
+/// assignment as the context, but still considers the assignment as a valid
+/// promotion/demotion point.
+
+/// A generic class to serve as the base type.
+class C<S> {
+  S cMethod(S x) => x;
+}
+
+/// An inference context C<S> constrains the first type variable of D but not
+/// the second.
+class D<S, T> extends C<S> {
+  S dMethod1(S x) => x;
+  T dMethod2(T x) => x;
+}
+
+/// Generic function which if inferred in a context C<A> with argument type B,
+/// should infer to mkD<A, B>
+D<S, T> mkD<S, T>(T x) => D();
+
+/// Generic function which if inferred in a context D<S0, S1> with argument type
+/// D<T0, T1>, should infer to useD<T0>.
+C<T> useD<T>(D<T, int> d) => d;
+
+void main() {
+  {
+    C<String> x = C();
+
+    // Inference uses C<String> as a downwards context, constraining S from
+    // mkD<S,T> to String.  Upwards inference constrains T to int.  D<String,
+    // int> is not a type of interest so no promotion should happen.
+    {
+      // y has the type of the RHS of the assignment
+      var y = x = mkD(3);
+
+      // x still has type C<String>
+      x.dMethod1("hello");
+      //^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_METHOD
+      // [cfe] The method 'dMethod1' isn't defined for the class 'C<String>'.
+      x.dMethod2(3);
+      //^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_METHOD
+      // [cfe] The method 'dMethod2' isn't defined for the class 'C<String>'.
+
+      var t0 = x.cMethod("hello");
+      t0.length;
+      t0.arglebargle; // t0 is not dynamic
+      // ^^^^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_GETTER
+      // [cfe] The getter 'arglebargle' isn't defined for the class 'String'.
+
+      // y has type D<String, int>
+      var t1 = y.dMethod1("hello");
+      t1.length;
+      t1.arglebargle; // t1 is not dynamic
+      // ^^^^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_GETTER
+      // [cfe] The getter 'arglebargle' isn't defined for the class 'String'.
+      var t2 = y.dMethod2(3);
+      t2.isEven;
+      t2.arglebargle; // t2 is not dynamic
+      // ^^^^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_GETTER
+      // [cfe] The getter 'arglebargle' isn't defined for the class 'int'.
+    }
+
+    // Establish D<String, int> as a type of interest
+    if (x is D<String, int>) {}
+    // Inference uses C<String> as a downwards context, constraining S from
+    // mkD<S,T> to String.  Upwards inference constrains T to int.  D<String,
+    // int> is a type of interest so promotion should happen.
+    {
+      // y has the type of the RHS of the assignment
+      var y = x = mkD(3);
+
+      // x has type D<String, int>
+      var t0 = x.dMethod1("hello");
+      t0.length;
+      t0.arglebargle; // t0 is not dynamic
+      // ^^^^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_GETTER
+      // [cfe] The getter 'arglebargle' isn't defined for the class 'String'.
+      var t1 = x.dMethod2(3);
+      t1.isEven;
+      t1.arglebargle; // t1 is not dynamic
+      // ^^^^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_GETTER
+      // [cfe] The getter 'arglebargle' isn't defined for the class 'int'.
+
+      // y has type D<String, int>
+      var t2 = y.dMethod1("hello");
+      t2.length;
+      t2.arglebargle; // t2 is not dynamic
+      // ^^^^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_GETTER
+      // [cfe] The getter 'arglebargle' isn't defined for the class 'String'.
+      var t3 = y.dMethod2(3);
+      t3.isEven;
+      t3.arglebargle; // t3 is not dynamic
+      // ^^^^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_GETTER
+      // [cfe] The getter 'arglebargle' isn't defined for the class 'int'.
+    }
+
+    // Inference should use D<String, int> as a downwards context, T from
+    // useD<T> to int.  The variable x as an argument has type D<String, int>
+    // which is assignable to D<String, int> so the call should have no error.
+    // C<String> is a type of interest, so x should be demoted after the call.
+    {
+      // y has the type of the RHS of the assignment
+      var y = x = useD(x);
+
+      // x has type C<String>, and not D<String, int>
+      x.dMethod1("hello");
+      //^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_METHOD
+      // [cfe] The method 'dMethod1' isn't defined for the class 'C<String>'.
+      x.dMethod2(3);
+      //^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_METHOD
+      // [cfe] The method 'dMethod2' isn't defined for the class 'C<String>'.
+
+      var t0 = x.cMethod("hello");
+      t0.length;
+      t0.arglebargle; // t0 is not dynamic
+      // ^^^^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_GETTER
+      // [cfe] The getter 'arglebargle' isn't defined for the class 'String'.
+
+      // C<String>, and not D<String, int>
+      y.dMethod1("hello");
+      //^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_METHOD
+      // [cfe] The method 'dMethod1' isn't defined for the class 'C<String>'.
+      y.dMethod2(3);
+      //^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_METHOD
+      // [cfe] The method 'dMethod2' isn't defined for the class 'C<String>'.
+      var t1 = y.cMethod("hello");
+      t1.length;
+      t1.arglebargle; // t0 is not dynamic
+      // ^^^^^^^^^^^
+      // [analyzer] STATIC_TYPE_WARNING.UNDEFINED_GETTER
+      // [cfe] The getter 'arglebargle' isn't defined for the class 'String'.
+    }
+  }
+}
diff --git a/tests/language/nnbd/typedef/typedef_error_test.dart b/tests/language/nnbd/typedef/typedef_error_test.dart
new file mode 100644
index 00000000000..f1bed377d69
--- /dev/null
+++ b/tests/language/nnbd/typedef/typedef_error_test.dart
@@ -0,0 +1,21 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Requirements=nnbd-weak
+
+/// Test that typedefs imported from opted out libraries are treated as
+/// non-nullable at the top level, with legacy components.
+
+import 'typedef_opted_out.dart';
+
+int? takesNonNullable(int x) {}
+void main() {
+  F f = null; // typedefs from opted out libraries are treated as non-nullable
+  //    ^^^^
+  // [analyzer] STATIC_TYPE_WARNING.INVALID_ASSIGNMENT
+  // [cfe] A value of type 'Null' can't be assigned to a variable of type 'int Function(int)'.
+
+  f = takesNonNullable; // F is int* Function(int*)
+  f(null); // F is int* Function(int*)
+}
diff --git a/tests/language/nnbd/typedef/typedef_opted_out.dart b/tests/language/nnbd/typedef/typedef_opted_out.dart
new file mode 100644
index 00000000000..1576bc620ce
--- /dev/null
+++ b/tests/language/nnbd/typedef/typedef_opted_out.dart
@@ -0,0 +1,7 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// @dart=2.7
+
+typedef F = int Function(int);