dart2js: Properly construct rti ast templates.

pkg/compiler/lib/src/js_backend/runtime_types.dart

Index: pkg/compiler/lib/src/js_backend/runtime_types.dart
diff --git a/pkg/compiler/lib/src/js_backend/runtime_types.dart b/pkg/compiler/lib/src/js_backend/runtime_types.dart
index 937703692faf049174143f2f3316d143294a8521..cee17622ac079808fec3a2dcf3cb79acf992b6c2 100644
--- a/pkg/compiler/lib/src/js_backend/runtime_types.dart
+++ b/pkg/compiler/lib/src/js_backend/runtime_types.dart
@@ -550,11 +550,19 @@ class RuntimeTypes {
     }
   }
 
+  /**
+   * Returns a [jsAst.Expression] representing the given [type]. Type
+   * variables are replaced by placeholders in the ast.
+   *
+   * [firstPlaceholderIndex] is the index to use for the first placeholder.
+   * This is useful if the returned [jsAst.Expression] is only part of a
+   * larger template. By default, indexing starts with 0.
+   */
   jsAst.Expression getTypeRepresentationWithPlaceholders(DartType type,
-      OnVariableCallback onVariable) {
+      OnVariableCallback onVariable, {int firstPlaceholderIndex : 0}) {
     // Create a type representation.  For type variables call the original
     // callback for side effects and return a template placeholder.
-    int positions = 0;
+    int positions = firstPlaceholderIndex;
     jsAst.Expression representation = getTypeRepresentation(type, (variable) {
       onVariable(variable);
       return new jsAst.InterpolatedExpression(positions++);