Don't allow statement-nodes in expression contexts.

lib/compiler/implementation/ssa/nodes.dart

Index: lib/compiler/implementation/ssa/nodes.dart
diff --git a/lib/compiler/implementation/ssa/nodes.dart b/lib/compiler/implementation/ssa/nodes.dart
index f0579a7886efa780218898f6704e9041c30e8a25..cfc8a8a32fedd0bedbec50e30aae75b2aeaa3e3d 100644
--- a/lib/compiler/implementation/ssa/nodes.dart
+++ b/lib/compiler/implementation/ssa/nodes.dart
@@ -992,7 +992,7 @@ class HBoolify extends HInstruction {
 abstract class HCheck extends HInstruction {
   HCheck(inputs) : super(inputs);
   HInstruction get checkedInput() => inputs[0];
-  final bool isStatement = true;
+  bool get isStatement() => true;
   void prepareGvn() {
     assert(!hasSideEffects());
     setUseGvn();
@@ -1002,7 +1002,7 @@ abstract class HCheck extends HInstruction {
 class HTypeGuard extends HCheck {
   final int state;
   final HType guardedType;
-  bool isOn = false;
+  bool isEnabled = false;
   int checkedInputIndex = 0;
 
   HTypeGuard(this.guardedType, this.state, List<HInstruction> env) : super(env);
@@ -1011,13 +1011,15 @@ class HTypeGuard extends HCheck {
   HInstruction get checkedInput() => guarded;
 
   HType computeTypeFromInputTypes() {
-    return isOn ? guardedType : guarded.propagatedType;
+    return isEnabled ? guardedType : guarded.propagatedType;
   }
 
-  HType get guaranteedType() => isOn ? guardedType : HType.UNKNOWN;
+  HType get guaranteedType() => isEnabled ? guardedType : HType.UNKNOWN;
 
   bool isControlFlow() => true;
 
+  bool get isStatement() => isEnabled;
+
   accept(HVisitor visitor) => visitor.visitTypeGuard(this);
   int typeCode() => 1;
   bool typeEquals(other) => other is HTypeGuard;