dart2js: Avoid inserting redundant HTypeKnown

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

Index: pkg/compiler/lib/src/ssa/nodes.dart
diff --git a/pkg/compiler/lib/src/ssa/nodes.dart b/pkg/compiler/lib/src/ssa/nodes.dart
index 915ec54aa46778730a8153fad263fc2d3c351195..fb1d40f385daba5a6b523b5186de8371c230bce0 100644
--- a/pkg/compiler/lib/src/ssa/nodes.dart
+++ b/pkg/compiler/lib/src/ssa/nodes.dart
@@ -1417,6 +1417,10 @@ class DominatedUses {
     }
   }
 
+  bool get isSingleton => _instructions.length == 1;
+
+  HInstruction get single => _instructions.single;
+
   void _addUse(HInstruction user, int inputIndex) {
     _instructions.add(user);
     _indexes.add(inputIndex);
@@ -3046,7 +3050,7 @@ class HTypeConversion extends HCheck {
 /// The [HTypeKnown] instruction marks a value with a refined type.
 class HTypeKnown extends HCheck {
   TypeMask knownType;
-  bool _isMovable;
+  final bool _isMovable;
 
   HTypeKnown.pinned(TypeMask knownType, HInstruction input)
       : this.knownType = knownType,
@@ -3066,6 +3070,8 @@ class HTypeKnown extends HCheck {
   bool isControlFlow() => false;
   bool canThrow() => false;
 
+  bool get isPinned => inputs.length == 1;
+
   HInstruction get witness => inputs.length == 2 ? inputs[1] : null;
 
   int typeCode() => HInstruction.TYPE_KNOWN_TYPECODE;