Plugin creates model element of custom, reference having, type without supplying an implementation

+review REVIEW-5236
Setting a model element property of a managed type to a non managed implementation is a runtime error

+review REVIEW-5236
Merge remote-tracking branch 'origin/master'

Conflicts:
	subprojects/core/src/main/groovy/org/gradle/internal/service/scopes/ProjectScopeServices.java
Post-merge cleanup.

+review REVIEW-5236