Support type variables as expressions.

sdk/lib/_internal/compiler/implementation/world.dart

Index: sdk/lib/_internal/compiler/implementation/world.dart
diff --git a/sdk/lib/_internal/compiler/implementation/world.dart b/sdk/lib/_internal/compiler/implementation/world.dart
index 66c32e66d19ea0aed8c563f855080ec3869f7072..31ad44c8188955c7db9cb861ed95862c196418fb 100644
--- a/sdk/lib/_internal/compiler/implementation/world.dart
+++ b/sdk/lib/_internal/compiler/implementation/world.dart
@@ -12,6 +12,10 @@ class World {
   final Map<ClassElement, Set<ClassElement>> rtiDependencies;
   final FullFunctionSet allFunctions;
 
+  // The set of classes that use one of their type variables as expressions
+  // to get the runtime type.
+  final Set<ClassElement> classesUsingTypeVariableExpression;
+
   // We keep track of subtype and subclass relationships in four
   // distinct sets to make class hierarchy analysis faster.
   final Map<ClassElement, Set<ClassElement>> subclasses =
@@ -30,6 +34,7 @@ class World {
         classesNeedingRti = new Set<ClassElement>(),
         rtiDependencies = new Map<ClassElement, Set<ClassElement>>(),
         allFunctions = new FullFunctionSet(compiler),
+            classesUsingTypeVariableExpression = new Set<ClassElement>(),
         this.compiler = compiler;
 
   void populate() {
@@ -130,6 +135,9 @@ class World {
         potentiallyAddForRti(variable.enclosingElement);
       }
     });
+    // Add the classes that need RTI because they use a type variable as
+    // expression.
+    classesUsingTypeVariableExpression.forEach(potentiallyAddForRti);
   }
 
   Iterable<ClassElement> commonSupertypesOf(ClassElement x, ClassElement y) {
@@ -179,6 +187,10 @@ class World {
     classes.add(dependency);
   }
 
+  void registerClassUsingVariableExpression(ClassElement cls) {
+    classesUsingTypeVariableExpression.add(cls);
+  }
+
   bool needsRti(ClassElement cls) {
     return classesNeedingRti.contains(cls) || compiler.enabledRuntimeType;
   }