Start implementing checked mode and tools support for using it.

lib/compiler/implementation/ssa/optimize.dart

Index: lib/compiler/implementation/ssa/optimize.dart
===================================================================
--- lib/compiler/implementation/ssa/optimize.dart	(revision 7269)
+++ lib/compiler/implementation/ssa/optimize.dart	(working copy)
@@ -130,7 +130,8 @@
     HInstruction input = inputs[0];
     if (input.isBoolean()) return input;
     // All values !== true are boolified to false.
-    if (input.propagatedType.isUseful()) {
+    Type type = input.propagatedType.computeType(compiler);
+    if (type !== null && type.element !== compiler.boolClass) {
       return graph.addConstantBool(false);
     }
     return node;
@@ -437,7 +438,7 @@
     }
 
     if (right.isConstantNull()) {
-      if (left.propagatedType.isUseful()) {
+      if (!left.propagatedType.canBeNull()) {

[floitsch, 2012/05/07 09:50:07]

I would prefer switching to the new semantics in a separate CL.

[ngeoffray, 2012/05/07 13:15:42]

This should have been if (leg.propagatedType.isPrimitive()) ...

         return graph.addConstantBool(false);
       } else {
         // TODO(floitsch): cache interceptors.
@@ -532,6 +533,7 @@
       }
     // TODO(karlklose): remove the hasTypeArguments check.
     } else if (expressionType.isUseful()
+               && !expressionType.canBeNull()

[floitsch, 2012/05/07 09:50:07]

you could move that check into the 'true' section.
if the is-check would be false, then 'null' can't change that.

[ngeoffray, 2012/05/07 13:15:42]

Done.

                && !compiler.universe.rti.hasTypeArguments(type)) {
       Type receiverType = expressionType.computeType(compiler);
       if (receiverType !== null) {
@@ -1054,6 +1056,10 @@
 
   void visitIs(HIs instruction) {
     HInstruction input = instruction.expression;
+    HType convertedType =
+        new HType.fromBoundedType(instruction.typeExpression, compiler);
+    if (convertedType === null) return;
+
     List<HInstruction> ifUsers = <HInstruction>[];
     List<HInstruction> notIfUsers = <HInstruction>[];
 
@@ -1069,9 +1075,6 @@
 
     if (ifUsers.isEmpty() && notIfUsers.isEmpty()) return;
 
-    HType convertedType =
-        new HType.fromBoundedType(instruction.typeExpression, compiler);
-
     for (HIf ifUser in ifUsers) {
       changeUsesDominatedBy(ifUser.thenBlock, input, convertedType);
       // TODO(ngeoffray): Also change uses for the else block on a HType