Fix build problems caused by on-demand imports

>>> Hi lambda people,
>>> I seem to have found a problem when using the last week's lamdba binary snapshot as the boot JDK for the build. The build works fine if the boot JDK is 7u9. I don't think I ever tried this combination before.
>>>
>>> Looks like there is a new java.util.Mapping in 8. So I think the guilty thing is use of java.util.* in src/share/classes/com/sun/tools/javac/code/Types.java?
>>>
>>> 26 package com.sun.tools.javac.code;
>>> 27
>>> 28 import java.lang.ref.SoftReference;
>>> 29 import java.util.*;
>>> 30