Make dart2js understand relative part-of in platform libraries.

pkg/compiler/lib/src/elements/modelx.dart

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index 2f40c9213783d019ba5029419a4cf0b194f33eae..5a241c6a2deff65b8ad2772e3905b970cf796fae 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -782,6 +782,10 @@ class CompilationUnitElementX extends ElementX
       String content = libraryReference.dartString.slowToString();
       Uri uri = this.script.readableUri.resolve(content);
       Uri expectedUri = library.canonicalUri;
+      // Also allow `string.dart` to refer to `dart:core` as `core.dart`.
+      if (library.isPlatformLibrary && !uri.isScheme("dart")) {
+        expectedUri = library.entryCompilationUnit.script.readableUri;
+      }
       if (uri != expectedUri) {
         // Consider finding a relative URI reference for the error message.
         reporter.reportWarningMessage(tag.name,