Compute correct constructor type for synthesized mixin constructors.

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 93665658b3571409762a1f4da892fb4d9306bb54..c33e7f0d0955a1fcf0635b2252c91b6292d2d8b9 100644
--- a/tests/compiler/dart2js/memory_compiler.dart
+++ b/tests/compiler/dart2js/memory_compiler.dart
@@ -84,7 +84,7 @@ Compiler compilerFor(Map<String,String> memorySourceFiles,
                       bool showDiagnostics: true}) {
   Uri script = currentDirectory.resolveUri(Platform.script);
   Uri libraryRoot = script.resolve('../../../sdk/');
-  Uri packageRoot = script.resolve('./packages/');
+  Uri packageRoot = currentDirectory.resolve('${Platform.packageRoot}/');
 
   MemorySourceFileProvider provider;
   var readStringFromUri;