Fix #1037

[odersky]

Review by @smarter

[smarter]

Wow, this is interesting, after 3686713 is applied, the occasional AbstractMethodError we were seeing in some run tests are now reproducible:

> rm *.class
> ./bin/dotc tests/run/t7475b.scala
> scala -J-Xbootclasspath/a:.:target/scala-2.11/classes/ Test
java.lang.AbstractMethodError: Method Test$$anon$5.x()I is abstract
        at Test$$anon$5.x(t7475b.scala)
        at C1.$init$(t7475b.scala:3)
        at Test$$anon$5.<init>(t7475b.scala:8)
        at Test$.main(t7475b.scala:8)
        at Test.main(t7475b.scala)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:497)
        at scala.reflect.internal.util.ScalaClassLoader$$anonfun$run$1.apply(ScalaClassLoader.scala:70)
        at scala.reflect.internal.util.ScalaClassLoader$class.asContext(ScalaClassLoader.scala:31)
        at scala.reflect.internal.util.ScalaClassLoader$URLClassLoader.asContext(ScalaClassLoader.scala:101)
        at scala.reflect.internal.util.ScalaClassLoader$class.run(ScalaClassLoader.scala:70)
        at scala.reflect.internal.util.ScalaClassLoader$URLClassLoader.run(ScalaClassLoader.scala:101)
        at scala.tools.nsc.CommonRunner$class.run(ObjectRunner.scala:22)
        at scala.tools.nsc.ObjectRunner$.run(ObjectRunner.scala:39)
        at scala.tools.nsc.CommonRunner$class.runAndCatch(ObjectRunner.scala:29)
        at scala.tools.nsc.ObjectRunner$.runAndCatch(ObjectRunner.scala:39)
        at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:65)
        at scala.tools.nsc.MainGenericRunner.run$1(MainGenericRunner.scala:87)
        at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:98)
        at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:103)
        at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)

[smarter]

LGTM