[dart2js] Always compute platform binaries.
[dart2js] Always compute platform binaries.

Currently, in order to change experiment flags between tests,
we rebuild the sdk. This is made more complicated by null safety
because now we may want to enable null safety while compiling
the sdk but not while compiling user code.

To simplify things, this cl changes dart2js to always use the
prebuilt sdk, and to assume that no experiment flags require
rebuilding the sdk.

Change-Id: Ia1bbdc88d191f1a71d1688685186457fcfba28aa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/142703
Commit-Queue: Joshua Litt <9f8daed1ab51574c9a48ffacb03e4a1adb4f880c@google.com>
Reviewed-by: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk