Use polyfil rather for Object.assign (#15822)

This gives a small code size saving and it makes our codebase
more readable.

The reason for this change is that I ran into an issue where I was
trying to use `objAssign` in more places and it became apparent
that MINIMAL_RUNTIME didn't define the `objAssign` helper.

Also, move the defintion of the polyfill inside of `#if POLYFILL` so
it can be disabled using `-sNO_POLYFILL`.

See #15823 #15938

Author: sbc100

src/polyfill/objassign.js

@@ -0,0 +1,33 @@
+// Object.assign polyfill from:
+// https://github.com/google/closure-compiler/blob/master/src/com/google/javascript/jscomp/js/es6/util/assign.js
+
+#if !POLYFILL
+assert(false, "this file should never be included unless POLYFILL is set");
+#endif
+
+/**
+ * Equivalent to the Object.assign() method, but guaranteed to be available for use in code
+ * generated by the compiler.
+ *
+ * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign
+ *
+ * Copies values of all enumerable own properties from one or more
+ * sources to the given target object, and returns the target.
+ *
+ * @final
+ * @param {!Object} target The target object onto which to copy.
+ * @param {...?Object} var_args The source objects.
+ * @return {!Object} The target object is returned.
+ */
+if (Object.assign === 'undefined') {
+  Object.assign = function(target, source) {
+    for (var i = 1; i < arguments.length; i++) {
+      var source = arguments[i];
+      if (!source) continue;
+      for (var key in source) {
+        if (source.hasOwnProperty(key)) target[key] = source[key];
+      }
+    }
+    return target;
+  };
+}

src/preamble.js

@@ -990,7 +990,7 @@ function instantiateSync(file, info) {
 // This does not work with nested objects that has prototypes, but it suffices for WasmSourceMap and WasmOffsetConverter.
 function resetPrototype(constructor, attrs) {
   var object = Object.create(constructor.prototype);
-  return objAssign(object, attrs);
+  return Object.assign(object, attrs);
 }
 #endif
 

src/shell.js

@@ -43,21 +43,12 @@ var Module = typeof {{{ EXPORT_NAME }}} !== 'undefined' ? {{{ EXPORT_NAME }}} :
 // modularize which creates a Promise for when the module is ready.
 #include "polyfill/promise.js"
 #endif
-#endif
 
 // See https://caniuse.com/mdn-javascript_builtins_object_assign
 #if MIN_CHROME_VERSION < 45 || MIN_EDGE_VERSION < 12 || MIN_FIREFOX_VERSION < 34 || MIN_IE_VERSION != TARGET_NOT_SUPPORTED || MIN_SAFARI_VERSION < 90000
-function objAssign(target, source) {
-  for (var key in source) {
-    if (source.hasOwnProperty(key)) {
-      target[key] = source[key];
-    }
-  }
-  return target;
-}
-#else
-var objAssign = Object.assign;
+#include "polyfill/objassign.js"
 #endif
+#endif // POLYFILL
 
 #if MODULARIZE
 // Set up the promise that indicates the Module is initialized
@@ -80,7 +71,7 @@ Module['ready'] = new Promise(function(resolve, reject) {
 // we collect those properties and reapply _after_ we configure
 // the current environment's defaults to avoid having to be so
 // defensive during initialization.
-var moduleOverrides = objAssign({}, Module);
+var moduleOverrides = Object.assign({}, Module);
 
 var arguments_ = [];
 var thisProgram = './this.program';
@@ -420,7 +411,7 @@ if (ENVIRONMENT_IS_NODE) {
 #endif
 
 // Merge back in the overrides
-objAssign(Module, moduleOverrides);
+Object.assign(Module, moduleOverrides);
 // Free the object hierarchy contained in the overrides, this lets the GC
 // reclaim data used e.g. in memoryInitializerRequest, which is a large typed array.
 moduleOverrides = null;

tests/other/metadce/hello_libcxx_O2.jssize

@@ -1 +1 @@
-96809
+96785

tests/other/metadce/hello_libcxx_O2_EVAL_CTORS.jssize

@@ -1 +1 @@
-96809
+96785

tests/other/metadce/hello_libcxx_O2_EVAL_CTORS_2.jssize

@@ -1 +1 @@
-96707
+96683

tests/other/metadce/hello_libcxx_O2_fexceptions.jssize

@@ -1 +1 @@
-110263
+110239

tests/other/metadce/hello_libcxx_O2_fexceptions_DEMANGLE_SUPPORT.jssize

@@ -1 +1 @@
-111250
+111226

tests/other/metadce/hello_world.jssize

@@ -1 +1 @@
-125520
+125497

tests/other/metadce/hello_world_O1.jssize

@@ -1 +1 @@
-58390
+58367