fixes #209, remove tree mutation from CodeChecker

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 40e8b5de16f29c0db9396e121f7750ad36448fa0..1749ca742cde6543c3bce6a52615e60543ad2c73 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -49,7 +49,7 @@ const DSETINDEX = 'dsetindex';
 const DCALL = 'dcall';
 const DSEND = 'dsend';
 
-class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
+class JSCodegenVisitor extends GeneralizingAstVisitor {
   final AbstractCompiler compiler;
   final CompilerOptions options;
   final TypeRules rules;
@@ -233,12 +233,6 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
 
   bool isPublic(String name) => !name.startsWith('_');
 
-  /// Conversions that we don't handle end up here.
-  @override
-  visitConversion(Conversion node) {
-    throw 'Unlowered conversion ${node.runtimeType}: $node';
-  }
-
   @override
   visitAsExpression(AsExpression node) {
     var from = getStaticType(node.expression);
@@ -1819,9 +1813,6 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
     if (expr is ParenthesizedExpression) {
       return _isNonNullableExpression(expr.expression);
     }
-    if (expr is Conversion) {
-      return _isNonNullableExpression(expr.expression);
-    }
     if (expr is SimpleIdentifier) {
       // Type literals are not null.
       Element e = expr.staticElement;