[VM] Only run FFI transformation when dart:ffi is imported

Before this CL the FFI transformation was always run (for target VM),
so if for instance running dart2js (from source), e.g. like
`out/ReleaseX64/dart -DDFE_VERBOSE=true pkg/compiler/bin/dart2js.dart --help`
one could see how it spends something like 120-140 ms, which is
something along the lines of 2.5% of the entire request processing
(i.e. compile and serialization).

This CL first checks if dart:ffi is imported at all. If it's not the
transformation is skipped.
When compiling dart2js (where, at least currently, dart:ffi is not used)
this skips the transformation, thus saving the 120-140 ms (or ~2.5%).

All measurements on my machine. Your mileage may vary.

TEST=Existing tests.

Change-Id: Ia5d3d7b989f549ca5da257660c204e8c59f15d4b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/185543
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>

pkg/vm/lib/target/vm.dart

@@ -21,6 +21,7 @@ import 'package:kernel/vm/constants_native_effects.dart'
 import '../transformations/call_site_annotator.dart' as callSiteAnnotator;
 import '../transformations/lowering.dart' as lowering
     show transformLibraries, transformProcedure;
+import '../transformations/ffi.dart' as ffiHelper show importsFfi;
 import '../transformations/ffi_definitions.dart' as transformFfiDefinitions
     show transformLibraries;
 import '../transformations/ffi_use_sites.dart' as transformFfiUseSites
@@ -153,17 +154,29 @@ class VmTarget extends Target {
         this, coreTypes, hierarchy, libraries, referenceFromIndex);
     logger?.call("Transformed mixin applications");
 
-    final ffiTransformerData = transformFfiDefinitions.transformLibraries(
-        component,
-        coreTypes,
-        hierarchy,
-        libraries,
-        diagnosticReporter,
-        referenceFromIndex,
-        changedStructureNotifier);
-    transformFfiUseSites.transformLibraries(component, coreTypes, hierarchy,
-        libraries, diagnosticReporter, ffiTransformerData, referenceFromIndex);
-    logger?.call("Transformed ffi annotations");
+    if (!ffiHelper.importsFfi(component, libraries)) {
+      logger?.call("Skipped ffi transformation");
+    } else {
+      // TODO(jensj/dacoharkes): We can probably limit the transformations to
+      // libraries that transitivley depend on dart:ffi.
+      final ffiTransformerData = transformFfiDefinitions.transformLibraries(
+          component,
+          coreTypes,
+          hierarchy,
+          libraries,
+          diagnosticReporter,
+          referenceFromIndex,
+          changedStructureNotifier);
+      transformFfiUseSites.transformLibraries(
+          component,
+          coreTypes,
+          hierarchy,
+          libraries,
+          diagnosticReporter,
+          ffiTransformerData,
+          referenceFromIndex);
+      logger?.call("Transformed ffi annotations");
+    }
 
     // TODO(kmillikin): Make this run on a per-method basis.
     bool productMode = environmentDefines["dart.vm.product"] == "true";

pkg/vm/lib/transformations/ffi.dart

@@ -570,3 +570,17 @@ class FfiTransformerData {
   FfiTransformerData(
       this.replacedGetters, this.replacedSetters, this.emptyStructs);
 }
+
+/// Checks if any library depends on dart:ffi.
+bool importsFfi(Component component, List<Library> libraries) {
+  Set<Library> allLibs = {...component.libraries, ...libraries};
+  final Uri dartFfiUri = Uri.parse("dart:ffi");
+  for (Library lib in allLibs) {
+    for (LibraryDependency dependency in lib.dependencies) {
+      if (dependency.targetLibrary.importUri == dartFfiUri) {
+        return true;
+      }
+    }
+  }
+  return false;
+}