Rewrite "const" declarations of Polymer class objects to "let"

With this change, const declarations of Polymer will no longer throw POLYMER_INVALID_DECLARATION error and they will be converted to let declarations instead.

Polymer pass can not handle const declarations of Polymer class objects and throws POLYMER_INVALID_DECLARATION error. The existing JS code which uses this code pattern  gets away with it because ClosureRewriteModule pass rewrites the code to assign to a qualified name, instead of making it a const declaration. (due to export inlining). Since we will be moving closureRewriteModule pass to run after Polymer pass, we need
to make the Polymer pass handle the const declarations of Polymer classes.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=258601781

src/com/google/javascript/jscomp/PolymerPass.java

@@ -18,13 +18,13 @@
 import static com.google.common.base.MoreObjects.toStringHelper;
 import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Preconditions.checkNotNull;
-import static com.google.javascript.jscomp.PolymerPassErrors.POLYMER_INVALID_DECLARATION;
 import static com.google.javascript.jscomp.PolymerPassErrors.POLYMER_INVALID_EXTENDS;
 import static com.google.javascript.jscomp.PolymerPassErrors.POLYMER_MISSING_EXTERNS;
 
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.javascript.jscomp.NodeTraversal.ExternsSkippingCallback;
+import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
 import com.google.javascript.rhino.IR;
 import com.google.javascript.rhino.JSDocInfo;
 import com.google.javascript.rhino.JSDocInfoBuilder;
@@ -130,8 +130,12 @@ public void visit(NodeTraversal traversal, Node node, Node parent) {
   private void rewritePolymer1ClassDefinition(Node node, Node parent, NodeTraversal traversal) {
     Node grandparent = parent.getParent();
     if (grandparent.isConst()) {
-      compiler.report(JSError.make(node, POLYMER_INVALID_DECLARATION));
-      return;
+      grandparent.setToken(Token.LET);
+      Node scriptNode = traversal.getCurrentScript();
+      if (scriptNode != null) {
+        NodeUtil.addFeatureToScript(scriptNode, Feature.LET_DECLARATIONS);
+      }
+      traversal.reportCodeChange();
     }
     PolymerClassDefinition def = PolymerClassDefinition.extractFromCallNode(
         node, compiler, globalNames);

src/com/google/javascript/jscomp/PolymerPassErrors.java

@@ -26,11 +26,6 @@ final class PolymerPassErrors {
           "The argument to Polymer() is not an obj lit or the Polymer 2 class does not have a"
               + " static getter named 'config'. Ignoring this definition.");
 
-  // Disallow 'const Foo = Polymer(...)' because the code the PolymerPass outputs will reassign
-  // Foo which is not allowed for 'const' variables.
-  static final DiagnosticType POLYMER_INVALID_DECLARATION = DiagnosticType.error(
-      "JSC_POLYMER_INVALID_DECLARATION", "A Polymer() declaration cannot use ''const''.");
-
   static final DiagnosticType POLYMER_INVALID_BEHAVIOR = DiagnosticType.error(
       "JSC_POLYMER_INVALID_BEHAVIOR", "A Polymer behavior may not include an ''is'' property.");
 

test/com/google/javascript/jscomp/IntegrationTest.java

@@ -767,18 +767,15 @@ public void testPolymer1() {
   }
 
   @Test
-  public void testConstPolymerElementNotAllowed() {
+  public void testConstPolymerElementAllowed() {
     CompilerOptions options = createCompilerOptions();
     options.setPolymerVersion(1);
     options.setWarningLevel(DiagnosticGroups.CHECK_TYPES, CheckLevel.ERROR);
     options.setLanguageIn(LanguageMode.ECMASCRIPT_2017);
     options.setLanguageOut(LanguageMode.ECMASCRIPT5);
     addPolymerExterns();
 
-    test(
-        options,
-        "const Foo = Polymer({ is: 'x-foo' });",
-        PolymerPassErrors.POLYMER_INVALID_DECLARATION);
+    testNoWarnings(options, "const Foo = Polymer({ is: 'x-foo' });");
   }
 
   private void addPolymer2Externs() {

test/com/google/javascript/jscomp/PolymerPassTest.java

@@ -20,7 +20,6 @@
 import static com.google.javascript.jscomp.PolymerPassErrors.POLYMER_CLASS_PROPERTIES_INVALID;
 import static com.google.javascript.jscomp.PolymerPassErrors.POLYMER_CLASS_PROPERTIES_NOT_STATIC;
 import static com.google.javascript.jscomp.PolymerPassErrors.POLYMER_DESCRIPTOR_NOT_VALID;
-import static com.google.javascript.jscomp.PolymerPassErrors.POLYMER_INVALID_DECLARATION;
 import static com.google.javascript.jscomp.PolymerPassErrors.POLYMER_INVALID_EXTENDS;
 import static com.google.javascript.jscomp.PolymerPassErrors.POLYMER_INVALID_PROPERTY;
 import static com.google.javascript.jscomp.PolymerPassErrors.POLYMER_MISSING_IS;
@@ -257,11 +256,15 @@ public void testLetTarget() {
 
   @Test
   public void testConstTarget() {
-    testError(
-        lines(
-            "const X = Polymer({",
-            "  is: 'x-element',",
-            "});"), POLYMER_INVALID_DECLARATION);
+    test(
+        lines("const X = Polymer({", "  is: 'x-element',", "});"),
+        lines(
+            "/**\n",
+            "* @constructor\n",
+            "* @extends {PolymerElement}\n",
+            "* @implements {PolymerXInterface}\n",
+            "*/\n",
+            "var X=function(){};X=Polymer(/** @lends {X.prototype} */ {is:\"x-element\"})"));
 
     test(
         lines(