Assocating each template node with a the generated TypeScript
generated overly verbose source maps. Changed to creating a
source map entry per unique source span instead of each
unique template ast node.
Fixes: #19537
PR Close#19578
This is important to not confuse users nor downstream tools that
consume our source maps. For generated content for which we don’t
have an original source file, we use the generated file now.
Fixes#19538
The source map does not currently work with the transformer pipeline.
It will be re-enabled after TypeScript 2.4 is made the min version.
To revert to the former compiler, use the `disableTransformerPipeline` in
tsconfig.json:
```
{
"angularCompilerOptions": {
"disableTransformerPipeline": true
}
}
```
