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..608e4eca72bd5120a788248d0e201d931a99ad52 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -782,6 +782,9 @@ class CompilationUnitElementX extends ElementX
       String content = libraryReference.dartString.slowToString();
       Uri uri = this.script.readableUri.resolve(content);

[Lasse Reichstein Nielsen, 2017/05/29 12:46:28]

This "readableUri", is that the package URI for a package: library?

[Johnni Winther, 2017/05/29 12:52:31]

Yes.

       Uri expectedUri = library.canonicalUri;
+      if (library.isPlatformLibrary && !uri.isScheme("dart")) {
+        expectedUri = library.entryCompilationUnit.script.readableUri;

[Johnni Winther, 2017/05/29 12:52:31]

Add a comment like:

// Allow 'string.dart' to refer to 'dart:core' as 'core.dart'.

[Lasse Reichstein Nielsen, 2017/05/29 13:12:03]

Done.

+      }
       if (uri != expectedUri) {
         // Consider finding a relative URI reference for the error message.
         reporter.reportWarningMessage(tag.name,