Change Java sourceLevel to default to 1.7.

The current JDT version erroneusly emits errors when compiling Guava
if the source level is set to 1.8. The problem seems to have been
already fixed (https://bugs.eclipse.org/bugs/show_bug.cgi?id=443596)
but there has been no "official" release of jdt.core yet.

Change-Id: I6ab263865660bd882bda108fab86f9a0f0e8f396
Allow property values to be restricted via command line flag.

The command line argument should be like this:
 [-setProperty name=value,value...]

-setProperty flag can be used multiple times to set
different properties.

If a property is set multiple times, the last one overwrites
the others.

Change-Id: I9df49c81790a4eb77f3698b7c8cdcafe6dcb5460
Significantly improve performance of String.trim().

This change uses simple for-loops and int arithmetics so
it's way faster [1] than the old implementation using regexps.
Code size increase is miserably small.

[1] http://jsperf.com/trim-performance-test/9

Change-Id: I4d2e25328e72586614b85fb91af0e259f82aebda
Improves error messaging for Enum.valueOf.

Change-Id: I0a5c6ad5fb70dc3e0cee747556188adcd8aa4451
Adds gwt27_28userApi.conf

Change-Id: I30b972ec71d3b3ebd5349641e83cbf6ab6a21706