dart2js: Fix SSA corner case with always breaking loop and failing assert

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

Index: sdk/lib/_internal/compiler/implementation/ssa/validate.dart
diff --git a/sdk/lib/_internal/compiler/implementation/ssa/validate.dart b/sdk/lib/_internal/compiler/implementation/ssa/validate.dart
index 5e9598e95b2c5acbc759aaca406254ba033768a9..8d98afe1fa3f6b244f48df512ddc977d852db93d 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/validate.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/validate.dart
@@ -92,6 +92,7 @@ class HValidator extends HInstructionVisitor {
     // corresponding predecessor block. Note that a block dominates
     // itself.
     block.forEachPhi((HPhi phi) {
+      assert(phi.inputs.length <= block.predecessors.length);
       for (int i = 0; i < phi.inputs.length; i++) {
         HInstruction input = phi.inputs[i];
         if (!input.block.dominates(block.predecessors[i])) {