Fix for dartbug.com/6036: the intersection of two different types is not always conflicting.

lib/compiler/implementation/ssa/codegen.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.
 
 part of ssa;
 
 class SsaCodeGeneratorTask extends CompilerTask {
 
   final JavaScriptBackend backend;
 
@@ skipping 1643 matching lines @@
     if (type != null) {
       world.registerFieldGetter(
           node.element.name, node.element.getLibrary(), type);
     }
   }
 
   // Determine if an instruction is a simple number computation
   // involving only things with guaranteed number types and a given
   // field.
   bool isSimpleFieldNumberComputation(HInstruction value, HFieldSet node) {
-    if (value.guaranteedType.union(HType.NUMBER) == HType.NUMBER) return true;
+    if (value.guaranteedType.union(HType.NUMBER, compiler) == HType.NUMBER) {
+      return true;
+    }
     if (value is HBinaryArithmetic) {
       return (isSimpleFieldNumberComputation(value.left, node) &&
               isSimpleFieldNumberComputation(value.right, node));
     }
     if (value is HFieldGet) return value.element == node.element;
     return false;
   }
 
   visitFieldSet(HFieldSet node) {
     String name = backend.namer.getName(node.element);
@@ skipping 1345 matching lines @@
   if (leftType.canBeNull() && rightType.canBeNull()) {
     if (left.isConstantNull() || right.isConstantNull() ||
         (leftType.isPrimitive() && leftType == rightType)) {
       return '==';
     }
     return null;
   } else {
     return '===';
   }
 }