Various constant checks.

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/body_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
index 633198a1025d6c468b9e5d079f054b3e7a298e36..258344083f6f9af9c13902181890308a3d8a739b 100644
--- a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
@@ -1546,8 +1546,13 @@ class BodyBuilder extends ScopeListener<JumpTarget> implements BuilderHelper {
     debugEvent("AsOperator");
     DartType type = pop();
     Expression expression = popForValue();
-    push(new KernelAsExpression(expression, type)
-      ..fileOffset = offsetForToken(operator));
+    if (constantExpressionRequired) {
+      push(buildCompileTimeError(
+          "Not a constant expression.", operator.charOffset));
+    } else {
+      push(new KernelAsExpression(expression, type)
+        ..fileOffset = offsetForToken(operator));
+    }
   }
 
   @override
@@ -1565,7 +1570,12 @@ class BodyBuilder extends ScopeListener<JumpTarget> implements BuilderHelper {
       typePromoter.handleIsCheck(isExpression, isInverted, operand.variable,
           type, functionNestingLevel);
     }
-    push(isExpression);
+    if (constantExpressionRequired) {
+      push(buildCompileTimeError(
+          "Not a constant expression.", operator.charOffset));
+    } else {
+      push(isExpression);
+    }
   }
 
   @override
@@ -2908,6 +2918,10 @@ class DelayedAssignment extends ContextAccessor {
   }
 
   Expression handleAssignment(bool voidContext) {
+    if (helper.constantExpressionRequired) {
+      return helper.buildCompileTimeError(
+          "Not a constant expression.", offsetForToken(token));
+    }
     if (identical("=", assignmentOperator)) {
       return accessor.buildAssignment(value, voidContext: voidContext);
     } else if (identical("+=", assignmentOperator)) {