Handle platform access through prefix in dart2dart

tests/compiler/dart2js/mirror_helper_test.dart

Index: tests/compiler/dart2js/mirror_helper_test.dart
diff --git a/tests/compiler/dart2js/mirror_helper_test.dart b/tests/compiler/dart2js/mirror_helper_test.dart
index f9e1647ae45df11ceadc6680fa3f2ae2f4df8100..e454c7e601bdd0f1f699459255871de1fef3d3bc 100644
--- a/tests/compiler/dart2js/mirror_helper_test.dart
+++ b/tests/compiler/dart2js/mirror_helper_test.dart
@@ -43,8 +43,8 @@ void testWithMirrorRenaming({bool minify}) {
     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;
@@ -65,8 +65,8 @@ void testWithoutMirrorRenaming({bool minify}) {
 
     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;