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

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library mock_compiler;
 
 import 'dart:collection';
 import 'dart:uri';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart' as api;
@@ skipping 51 matching lines @@
     operator[](index) {}
     operator[]=(index, value) {}
     var add;
   }
   class JSString {
     var length;
     operator[](index) {}
     toString() {}
   }
   class JSNumber {
-    operator-() {}
-    operator +(other) {}
-    operator -(other) {}
-    operator ~/(other) {}
-    operator /(other) {}
-    operator *(other) {}
-    operator <<(other) {}
-    operator >>(other) {}
-    operator |(other) {}
-    operator &(other) {}
-    operator ^(other) {}
-    operator >(other) {}
-    operator >=(other) {}
-    operator <(other) {}
-    operator <=(other) {}
-    operator ==(other) {}
-    operator %(other) {}
+    // All these methods return a number to please type inferencing.
+    operator-() => (this is JSInt) ? 42 : 42.0;
+    operator +(other) => (this is JSInt) ? 42 : 42.0;
+    operator -(other) => (this is JSInt) ? 42 : 42.0;
+    operator ~/(other) => 42;
+    operator /(other) => (this is JSInt) ? 42 : 42.0;
+    operator *(other) => (this is JSInt) ? 42 : 42.0;
+    operator %(other) => (this is JSInt) ? 42 : 42.0;
+    operator <<(other) => 42;
+    operator >>(other) => 42;
+    operator |(other) => 42;
+    operator &(other) => 42;
+    operator ^(other) => 42;
+
+    operator >(other) => true;
+    operator >=(other) => true;
+    operator <(other) => true;
+    operator <=(other) => true;
+    operator ==(other) => true;
   }
   class JSInt {
   }
   class JSDouble {
   }
   class JSNull {
   }
   class JSBool {
   }
   class JSFunction {
@@ skipping 234 matching lines @@
   operator []=(Node node, Element element) {
     map[node] = element;
   }
 
   operator [](Node node) => map[node];
 
   void remove(Node node) {
     map.remove(node);
   }
 }