Use type mask to figure out if we can constant fold boolify operations.

sdk/lib/_internal/compiler/implementation/ssa/optimize.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
diff --git a/sdk/lib/_internal/compiler/implementation/ssa/optimize.dart b/sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
index 7a41583029fa382f8aed202446162e1da390dbf4..df026d44805e13c757061e3c921835b55ae19a51 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
@@ -183,8 +183,12 @@ class SsaConstantFolder extends HBaseVisitor implements OptimizationPhase {
     HInstruction input = inputs[0];
     HType type = input.instructionType;
     if (type.isBoolean()) return input;
-    // All values !== true are boolified to false.
-    if (!type.isBooleanOrNull() && !type.isUnknown()) {
+    // All values that cannot be 'true' are boolified to false.
+    DartType booleanType = backend.jsBoolClass.computeType(compiler);

[karlklose, 2013/03/05 09:29:21]

You could use '.rawType' here.

Long term I think we want to get away from using computeType, because it has led
to confusion about what the returned type means.  For interface types we have
raw type (which is what most of the backend wants to use) and thisType (which
contains the type variables and is most used in resolver and type-checker).

+    TypeMask mask = type.computeMask(compiler);
+    // TODO(kasperl): Get rid of the null check here once all HTypes
+    // have a proper mask.
+    if (mask != null && !mask.contains(booleanType, compiler)) {
       return graph.addConstantBool(false, constantSystem);
     }
     return node;