Fix a ClassCastException in InferJSDocInfo

When a class node has an UnknownType rather than a FunctionType (this can happen when the class is immediately inside a CAST node to unknown, though I'm not positive that that's the root cause), trying to cast the type to FunctionType fails.  In general, casting JSType is almost always the wrong thing to do.  Instead, JSType.toMaybeFunctionType is much safer.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=206239547

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

@@ -180,7 +180,7 @@ public void visit(NodeTraversal t, Node n, Node parent) {
 
           final ObjectType owningType;
           if (parent.isClassMembers()) {
-            FunctionType ctorType = (FunctionType) parent.getParent().getJSType();
+            FunctionType ctorType = JSType.toMaybeFunctionType(parent.getParent().getJSType());
             if (ctorType == null) {
               return;
             }

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

@@ -1041,6 +1041,20 @@ public void testJSDocIsPropagatedToTypeFromObjectLiteralPrototype() {
     assertEquals("Property d.", prototype.getOwnPropertyJSDocInfo("d").getBlockDescription());
   }
 
+  public void testClassWithUnknownTypeDoesNotCrash() {
+    // Code that looks like this and @suppresses {checkTypes} is generated by tsickle.
+    ignoreWarnings(TypeCheck.CONFLICTING_EXTENDED_TYPE);
+    enableTypeCheck();
+    testSame(
+        lines(
+            "/** @param {?} base */",
+            "function mixin(base) {",
+            "  return (/** @type {?} */ ((class extends base {",
+            "    /** @param {...?} args */ constructor(...args) {}",
+            "  })));",
+            "}"));
+  }
+
   /** Returns the inferred type of the reference {@code name} anywhere in the AST. */
   private JSType inferredTypeOfName(String name) {
     return inferredTypeOfLocalName(name, getLastCompiler().getRoot());