Start work on a non-complete type inferrer. Currently only analyzes return types.

sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart

Index: sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart	(revision 18311)
+++ sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart	(working copy)
@@ -391,7 +391,7 @@
  * A task which conservatively infers a [ConcreteType] for each sub expression
  * of the program. The entry point is [analyzeMain].
  */
-class ConcreteTypesInferrer {
+class ConcreteTypesInferrer extends TypesInferrer {
   static final bool LOG_FAILURES = true;
 
   final String name = "Type inferrer";
@@ -686,12 +686,15 @@
   /**
    * Get the inferred concrete type of [node].
    */
-  ConcreteType getConcreteTypeOfNode(Node node) => inferredTypes[node];
+  ConcreteType getConcreteTypeOfNode(Element owner, Node node) {
+    return inferredTypes[node];
+  }
 
   /**
-   * Get the inferred concrete type of [parameter].
+   * Get the inferred concrete type of [element].
    */
-  ConcreteType getConcreteTypeOfParameter(VariableElement parameter) {
+  ConcreteType getConcreteTypeOfElement(Element element) {
+    if (!element.isParameter()) return null;
     return inferredParameterTypes[parameter];
   }