ng add @angular/pwa doesn't correctly edit the index.html

## ng add @angular/pwa doesn't correctly edit the index.html

### Bug Report or Feature Request (mark with an `x`)
```
- [x] bug report -> please search issues before submitting
- [ ] feature request
```

### Command (mark with an `x`)
```
- [ ] new
- [ ] build
- [ ] serve
- [ ] test
- [ ] e2e
- [ ] generate
- [x] add
- [ ] update
- [ ] lint
- [ ] xi18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc
```

### Versions

```
     _                      _                 ____ _     ___
    / \   _ __   __ _ _   _| | __ _ _ __     / ___| |   |_ _|
   / △ \ | '_ \ / _` | | | | |/ _` | '__|   | |   | |    | |
  / ___ \| | | | (_| | |_| | | (_| | |      | |___| |___ | |
 /_/   \_\_| |_|\__, |\__,_|_|\__,_|_|       \____|_____|___|
                |___/


Angular CLI: 6.1.4
Node: 8.11.3
OS: win32 x64
Angular: 6.1.3
... animations, common, compiler, compiler-cli, core, forms
... http, language-service, platform-browser
... platform-browser-dynamic, router, service-worker

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.7.4
@angular-devkit/build-angular     0.7.4
@angular-devkit/build-optimizer   0.7.4
@angular-devkit/build-webpack     0.7.4
@angular-devkit/core              0.7.4
@angular-devkit/schematics        0.7.4
@angular/cli                      6.1.4
@angular/pwa                      0.7.4
@ngtools/webpack                  6.1.4
@schematics/angular               0.7.4
@schematics/update                0.7.4
rxjs                              6.2.2
typescript                        2.7.2
webpack                           4.9.2
```

### Repro steps

```
ng new my-app --style=scss --routing
cd my-app
ng add @angular/pwa
```
## The log given by the failure

### BEFORE ng add @angular/pwa
```
<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>my-app</title>
  <base href="/">

  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="icon" type="image/x-icon" href="favicon.ico">
</head>
<body>
  <app-root></app-root>
</body>
</html>
```

### AFTER ng add @angular/pwa
```
<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>my-app</title>
  <base href="/">

  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="icon" type="image/x-icon" href="favicon.ico">
  <link rel="manifest" href="manifest.json">
  <meta name="theme-color" content="#1976d2">
</head>
<body>
  <app-root></app-root>
  <noscript>Please enable JavaScript to continue using this application.</noscript>
</head>                     <------|
<body>                      <------|-----INCORRECTLY TAGS ADDED
  <app-root></app-root>     <------|
</body>
</html>
```

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ng add @angular/pwa doesn't correctly edit the index.html #11942