Fix issue 6472 that exhibited a few crashing bugs in dart2js.

lib/compiler/implementation/ssa/optimize.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;
 
 abstract class OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
@@ skipping 124 matching lines @@
       HInstruction replacement = instruction.accept(this);
       if (!identical(replacement, instruction)) {
         if (!replacement.isInBasicBlock()) {
           // The constant folding can return an instruction that is already
           // part of the graph (like an input), so we only add the replacement
           // if necessary.
           block.addAfter(instruction, replacement);
         }
         block.rewrite(instruction, replacement);
         block.remove(instruction);
-        // If the replacement instruction does not know its type or
-        // source element yet, use the type and source element of the
+
+        // If we can replace [instruction] with [replacement], then
+        // [replacement]'s type can be narrowed.
+        types[replacement] =
+           types[replacement].intersection(types[instruction], compiler);
+        // If the replacement instruction does not know its

[kasperl, 2012/11/02 11:09:41]

Add newline before the comment here.

[ngeoffray, 2012/11/02 11:15:31]

Done.

+        // source element, use the source element of the
         // instruction.
-        if (!types[replacement].isUseful()) {
-          types[replacement] = types[instruction];
-        }
         if (replacement.sourceElement == null) {
           replacement.sourceElement = instruction.sourceElement;
         }
         if (replacement.sourcePosition == null) {
           replacement.sourcePosition = instruction.sourcePosition;
         }
       }
       instruction = next;
     }
   }
@@ skipping 1184 matching lines @@
     }
 
     // For other fields having setters in the generative constructor body, set
     // the type to UNKNOWN to avoid relying on the type set in the initializer
     // list.
     allSetters.forEach((Element element) {
       backend.registerFieldConstructor(element, HType.UNKNOWN);
     });
   }
 }