Port of f24ea59f74552113f8e8586d2d69045254512aa to 4.4.x
Closure no longer needs to have the imports rewritten so avoid
rewriting as this can cause issues when the source directory
structure differs from what is deployed.
Fixes: #17171
PR Close#19579
Now converts shorthand imports for every TypeScript target. Tsickle is able to expand index shorthand imports for every TypeScript target and module possibility.
Expanding shorthand imports for CommonJS modules is also helpful when testing in the browser. Module loaders like SystemJS are not able to understand directory imports (or index shorthand imports)
Update tsickle to version 0.21.6 which fixes a bug where input source maps which specified filenames differently than the names supplied to tsc didn't get composed with tsc's source maps. Also adds a test that the bug was fixed.
In order for tsickle's new support for input source maps to work, the tsickleCompilerHost must be used for the initial load of source files, since that's when the inline source maps are read and stripped.
Tsickle transforms typescript code, which can change the location of code.
This can cause issues such as runtime stack traces reporting that errors
were raised on the incorrect line in the orginal source. This change replaces
the DecoratorDownlevelCompilerHost and the TsickleCompilerHost with tsickle's
TsickleCompilerHost, which automatically composes tsickle's source maps with
typescript's source maps, so that line numbers are correctly reported at
runtime.
PR Close#14150
