Emit warning on import of dart:mirrors.

tests/compiler/dart2js/memory_compiler.dart

Index: tests/compiler/dart2js/memory_compiler.dart
diff --git a/tests/compiler/dart2js/memory_compiler.dart b/tests/compiler/dart2js/memory_compiler.dart
index de75d6c62322e164edafa8a8261cf6d631f7982d..88bc8e217c5f6fd8b1ad541ed206829bdb2750db 100644
--- a/tests/compiler/dart2js/memory_compiler.dart
+++ b/tests/compiler/dart2js/memory_compiler.dart
@@ -10,13 +10,16 @@ import 'package:compiler/src/dart2jslib.dart'
        show NullSink;
 
 import 'package:compiler/compiler.dart'
-       show Diagnostic, DiagnosticHandler, CompilerOutputProvider;
+    show Diagnostic, DiagnosticHandler, CompilerOutputProvider;
 
 import 'dart:async';
 
 import 'package:compiler/src/mirrors/source_mirrors.dart';
 import 'package:compiler/src/mirrors/analyze.dart';
 
+import 'package:compiler/src/library_loader.dart'
+    show LoadedLibraries;
+
 class DiagnosticMessage {
   final Uri uri;
   final int begin;
@@ -183,7 +186,7 @@ Compiler compilerFor(Map<String,String> memorySourceFiles,
     });
     // TODO(johnniwinther): Assert that no libraries are loaded lazily from
     // this call.
-    compiler.onLibrariesLoaded(copiedLibraries);
+    compiler.onLibrariesLoaded(new MemoryLoadedLibraries(copiedLibraries));
 
     compiler.symbolConstructor = cachedCompiler.symbolConstructor;
     compiler.mirrorSystemClass = cachedCompiler.mirrorSystemClass;
@@ -229,6 +232,27 @@ Compiler compilerFor(Map<String,String> memorySourceFiles,
   return compiler;
 }
 
+class MemoryLoadedLibraries implements LoadedLibraries {
+  final Map copiedLibraries;
+
+  MemoryLoadedLibraries(this.copiedLibraries);
+
+  @override
+  bool containsLibrary(Uri uri) => copiedLibraries.containsKey(uri);
+
+  @override
+  void forEachImportChain(f) {}
+
+  @override
+  void forEachLibrary(f) {}
+
+  @override
+  getLibrary(Uri uri) => copiedLibraries[uri];
+
+  @override
+  Uri get rootUri => null;
+}
+
 Future<MirrorSystem> mirrorSystemFor(Map<String,String> memorySourceFiles,
                                      {DiagnosticHandler diagnosticHandler,
                                       List<String> options: const [],