Undo optimization in cases where NaN might be involved.

lib/compiler/implementation/ssa/codegen.dart

Index: lib/compiler/implementation/ssa/codegen.dart
diff --git a/lib/compiler/implementation/ssa/codegen.dart b/lib/compiler/implementation/ssa/codegen.dart
index b486e0c62e1453be31363255855eb58daf207840..66239fb08a953bbdc060bf818b96726c38d203eb 100644
--- a/lib/compiler/implementation/ssa/codegen.dart
+++ b/lib/compiler/implementation/ssa/codegen.dart
@@ -1689,7 +1689,12 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       use(input.inputs[0], JSPrecedence.EQUALITY_PRECEDENCE);
       buffer.add(' !== true');
       endExpression(JSPrecedence.EQUALITY_PRECEDENCE);
-    } else if (isBuiltinRelational(input) && isGenerateAtUseSite(input)) {
+    } else if (isBuiltinRelational(input) &&
+               isGenerateAtUseSite(input) &&
+               !input.inputs[0].isDouble() &&
+               !input.inputs[0].isTypeUnknown()) {

[ngeoffray, 2012/05/10 10:41:13]

I think this will cover 'conflicting'. What you should use is
input.inputs[0].propagatedType.isUseful()

+      // This optimization doesn't work for NaN, so we don't do it if the
+      // type isn't known to be non-Double.
       Map<String, String> inverseOperator = const <String>{
         "==" : "!=",
         "!=" : "==",