Handle platform access through prefix in dart2dart

tests/compiler/dart2js/mirror_helper_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import "package:async_helper/async_helper.dart";
 import 'memory_compiler.dart' show compilerFor;
 import 'package:compiler/src/apiimpl.dart' show
     Compiler;
@@ skipping 25 matching lines @@
       compiler.runCompiler(Uri.parse('memory:main.dart')).then((_) => compiler);
 }
 
 void testWithMirrorRenaming({bool minify}) {
   asyncTest(() => runCompiler(useMirrorHelperLibrary: true, minify: minify).
       then((Compiler compiler) {
 
     DartBackend backend = compiler.backend;
     MirrorRenamerImpl mirrorRenamer = backend.mirrorRenamer;
     Map<Node, String> renames = backend.placeholderRenamer.renames;
-    Set<LibraryElement> imports =
-        backend.placeholderRenamer.platformImports;
+    Iterable<LibraryElement> imports =
+        backend.placeholderRenamer.platformImports.keys;
 
     FunctionExpression node = backend.memberNodes.values.first.first;
     Block block = node.body;
     ExpressionStatement getNameFunctionNode = block.statements.nodes.head;
     Send send = getNameFunctionNode.expression;
 
     Expect.equals(renames[mirrorRenamer.getNameFunctionNode.name],
                   renames[send.selector]);
     Expect.equals("",
                   renames[send.receiver]);
     Expect.equals(1, imports.length);
   }));
 }
 
 void testWithoutMirrorRenaming({bool minify}) {
   asyncTest(() => runCompiler(useMirrorHelperLibrary: false, minify: minify).
       then((Compiler compiler) {
 
     DartBackend backend = compiler.backend;
     Map<Node, String> renames = backend.placeholderRenamer.renames;
-    Set<LibraryElement> imports =
-        backend.placeholderRenamer.platformImports;
+    Iterable<LibraryElement> imports =
+        backend.placeholderRenamer.platformImports.keys;
     FunctionExpression node = backend.memberNodes.values.first.first;
     Block block = node.body;
     ExpressionStatement getNameFunctionNode = block.statements.nodes.head;
     Send send = getNameFunctionNode.expression;
 
     Expect.isFalse(renames.containsKey(send.selector));
     Expect.equals(1, imports.length);
   }));
 }
 
 const MEMORY_SOURCE_FILES = const <String, String> {
   'main.dart': """
 import 'dart:mirrors';
 
 class Foo {
   noSuchMethod(Invocation invocation) {
     MirrorSystem.getName(invocation.memberName);
   }
 }
 
 void main() {
   new Foo().fisk();
 }
 """};