dart2js: Introduce HGetLength to replace strange use of HFieldGet

pkg/compiler/lib/src/ssa/ssa_tracer.dart

Index: pkg/compiler/lib/src/ssa/ssa_tracer.dart
diff --git a/pkg/compiler/lib/src/ssa/ssa_tracer.dart b/pkg/compiler/lib/src/ssa/ssa_tracer.dart
index c76b592089e2658516a7397085197ce8b65fb8d3..d06c00ee32c2b61d67cc450eb3690704e9b78669 100644
--- a/pkg/compiler/lib/src/ssa/ssa_tracer.dart
+++ b/pkg/compiler/lib/src/ssa/ssa_tracer.dart
@@ -238,6 +238,10 @@ class HInstructionStringifier implements HVisitor<String> {
     }
   }
 
+  String visitGetLength(HGetLength node) {
+    return 'GetLength: ${temporaryId(node.receiver)}';
+  }
+
   String visitLocalGet(HLocalGet node) {
     String localName = node.variable.name;
     return 'LocalGet: ${temporaryId(node.local)}.$localName';