Minor fixes.

1. This is a @Deprecated method and very meaningless to modify, but I changed internal cast of Collection.multiply(Number) from List to Collection. This was a bug.
2. Removed meaningless @SupressWarning of Collection.grep().
3. << to &lt;&lt; in JavaDoc.
4. Fixed some wrong JavaDoc.
GROOVY-6961: fix wrong generics type extraction for extension methods by using the custom declared class instead of backing node
Merge pull request #489 from yukoba/MinorFix

Minor fixes
Introduce a BytecodeDumper class for reuse in tests
GROOVY-6968: Improvements to @ASTTest