metalet with postfix improvement

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index 062370e7ea52277cebcd586d6dfde0d03548ff8c..c87123fe86b906f4f16f8ee9f1be0fa7c0662f0a 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -1543,6 +1543,27 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
     }
   }
 
+  JS.Expression _assignableNotNull(Expression expr) {
+    if (expr is IndexExpression) {
+      IndexExpression i = expr;
+      return js.call(
+          'dart.index(#, #).notNull', [_visit(i.target), _visit(i.index)]);
+    } else if (expr is PropertyAccess) {
+      PropertyAccess p = expr;
+      return js.call('dart.prop(#, #).notNull', [
+        _visit(p.target),
+        _emitMemberName(p.propertyName.name)
+      ]);
+    } else if (expr is PrefixedIdentifier) {
+      PrefixedIdentifier p = expr;
+      return js.call('dart.prop(#, #).notNull', [
+        _visit(p.prefix),
+        _emitMemberName(p.identifier.name)
+      ]);
+    }
+    return null;
+  }
+
   @override
   JS.Expression visitBinaryExpression(BinaryExpression node) {
     var op = node.operator;
@@ -1747,9 +1768,16 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
 
     var dispatchType = rules.getStaticType(expr);
     if (unaryOperationIsPrimitive(dispatchType)) {
+      JS.Expression lvalue;
       if (_isNonNullableExpression(expr)) {
-        return js.call('#$op', _visit(expr));
+        // TODO(jmesserly): ensure that expr is generated as a lvalue.
+        // I think this works today because if the result type is primitive,
+        // it can't be a dynamic call. But worth checking.
+        lvalue = _visit(expr);
+      } else {
+        lvalue = _assignableNotNull(expr);
       }
+      if (lvalue != null) return new JS.Postfix(op.lexeme, lvalue);
     }
 
     assert(op.lexeme == '++' || op.lexeme == '--');