[theme] Use isPlainObject to avoid dropping prototypes (mui#12100)

* use isPlainObject with deepclone to avoid droping prototypes

* revert yarn.lock

* missing newline

* increase size-limit

.size-limit.js

@@ -21,13 +21,13 @@ module.exports = [
     name: 'The initial cost people pay for using one component',
     webpack: true,
     path: 'packages/material-ui/build/Paper/index.js',
-    limit: '17.6 KB',
+    limit: '17.8 KB',
   },
   {
     name: 'The size of all the modules of material-ui.',
     webpack: true,
     path: 'packages/material-ui/build/index.js',
-    limit: '94.3 KB',
+    limit: '94.5 KB',
   },
   {
     name: 'The main bundle of the docs',

packages/material-ui/package.json

@@ -44,6 +44,7 @@
     "deepmerge": "^2.0.1",
     "dom-helpers": "^3.2.1",
     "hoist-non-react-statics": "^2.5.0",
+    "is-plain-object": "^2.0.4",
     "jss": "^9.3.3",
     "jss-camel-case": "^6.0.0",
     "jss-default-unit": "^8.0.2",

packages/material-ui/src/styles/createMuiTheme.js

@@ -1,6 +1,7 @@
 // @flow
 
 import deepmerge from 'deepmerge'; // < 1kb payload overhead when lodash/merge is > 3kb.
+import isPlainObject from 'is-plain-object';
 import warning from 'warning';
 import createBreakpoints from './createBreakpoints';
 import createMixins from './createMixins';
@@ -42,6 +43,9 @@ function createMuiTheme(options: Object = {}) {
         zIndex,
       },
       other,
+      {
+        isMergeableObject: isPlainObject,
+      },
     ),
   };
 

packages/material-ui/src/styles/createMuiTheme.test.js

@@ -77,6 +77,9 @@ describe('createMuiTheme', () => {
         MuiButtonBase: {
           disableRipple: true,
         },
+        MuiPopover: {
+          container: document.createElement('div'),
+        },
       };
       const muiTheme = createMuiTheme({ props });
       assert.deepEqual(muiTheme.props, props);