GroovyShell/Grape cannot use Extension Modules that extend Closure

@Grab('org.elasticsearch:elasticsearch-groovy:1.4.2')
def printClosure(Closure closure) {
  println closure.asJsonString()
}

printClosure { field = "simple" }

The extension method's signature is static String asJsonString(Closure self) (https://github.com/elasticsearch/elasticsearch-groovy/blob/v1.4.2/src/main/groovy/org/elasticsearch/groovy/common/xcontent/XContentBuilderExtensions.groovy#L97).

$ groovy --indy Test.groovy
Caught: groovy.lang.MissingMethodException: No signature of method: Test.asJsonString() is applicable for argument types: () values: []
Possible solutions: toString(), toString()
groovy.lang.MissingMethodException: No signature of method: Test.asJsonString() is applicable for argument types: () values: []
Possible solutions: toString(), toString()
	at Test.printClosure(Test.groovy:3)
	at Test.run(Test.groovy:6)

Tested on both a Mac and Linux (with and without --indy does not matter):

• Groovy Version: 2.3.9 JVM: 1.8.0_25 Vendor: Oracle Corporation OS: Mac OS X
• Groovy Version 2.3.9 JVM: 1.7.0_65 Vendor: Oracle Corporation OS: Linux

However, if I don't use Grape/the GroovyShell, then the same code works with the same dependencies. I always use Gradle to build my code, so it wasn't until a user pointed this issue out that I became aware of it.

This relates to GROOVY-6557. On my Mac, I was always able to reproduce GROOVY-6447, but my core issue appears to relate to Closure extension methods across multiple platforms (https://github.com/elasticsearch/elasticsearch-groovy/issues/19).