fix(compiler): allow tsc-wrapped to be compile with TypeScript 2.0

[chuckjaz]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[x] Bugfix

What is the current behavior? (You can also link to an open issue here)

tsc-watch CompilerHost implementations don't implement the newly required method getDirectories().

What is the new behavior?

Implemented getDirectories() in DelegatingCompilerHost to allow using compiling tsc-watch with TypeScript 2.0's .d.ts

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

[hansl]

LGTM. Can confirm this fixes our build problems with TS2 Beta. 👍

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