Make Analyzer, VM and dart2js accept URI strings as part-of library identifier.

pkg/compiler/lib/src/diagnostics/messages.dart

Index: pkg/compiler/lib/src/diagnostics/messages.dart
diff --git a/pkg/compiler/lib/src/diagnostics/messages.dart b/pkg/compiler/lib/src/diagnostics/messages.dart
index 626d8fda1e62625240e0c6a8299ab2e77c720d92..834cda536044011ea6458ac9a0a30c170deafc38 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -244,6 +244,7 @@ enum MessageKind {
   LIBRARY_NOT_FOUND,
   LIBRARY_NOT_SUPPORTED,
   LIBRARY_TAG_MUST_BE_FIRST,
+  LIBRARY_URI_MISMATCH,
   MAIN_HAS_PART_OF,
   MAIN_NOT_A_FUNCTION,
   MAIN_WITH_EXTRA_PARAMETER,
@@ -2029,6 +2030,26 @@ part of lib.bar;
             }
           ]),
 
+      MessageKind.LIBRARY_URI_MISMATCH: const MessageTemplate(
+          MessageKind.LIBRARY_URI_MISMATCH,
+          "Expected URI of library '#{libraryUri}'.",
+          howToFix: "Try changing the directive to 'part of "
+                    "\"#{libraryUri}\";'.",
+          examples: const [
+            const {
+              'main.dart': """
+library lib.foo;
+
+part 'part.dart';
+
+main() {}
+""",
+              'part.dart': """
+part of 'not-main.dart';
+"""
+            }
+          ]),
+
       MessageKind.MISSING_LIBRARY_NAME: const MessageTemplate(
           MessageKind.MISSING_LIBRARY_NAME,
           "Library has no name. Part directive expected library name "