perf: MainTest#checkModelIsTree needs less memory (#1596)

src/test/java/spoon/test/main/MainTest.java

@@ -347,17 +347,25 @@ protected void exit(CtElement e) {
 		assertEquals("All parents have to be consistent", 0, inconsistentParents.size());
 	}
 
+
+
 	/*
 	 * contract: each element is used only once
 	 * For example this is always true: field.getType() != field.getDeclaringType()
 	 */
 	private void checkModelIsTree(CtPackage rootPackage) {
+		Exception dummyException = new Exception("STACK");
 		PrinterHelper problems = new PrinterHelper(rootPackage.getFactory().getEnvironment());
 		Map<CtElement, Exception> allElements = new IdentityHashMap<>();
 		rootPackage.filterChildren(null).forEach((CtElement ele) -> {
-			Exception secondStack = new Exception("STACK");
+			//uncomment this line to get stacktrace of real problem. The dummyException is used to avoid OutOfMemoryException
+//			Exception secondStack = new Exception("STACK");
+			Exception secondStack = dummyException;
 			Exception firstStack = allElements.put(ele, secondStack);
 			if (firstStack != null) {
+				if(firstStack == dummyException) {
+					Assert.fail("The Spoon model is not a tree. The " + ele.getClass().getSimpleName() + ":" + ele.toString() + " is shared");
+				}
 				//the element ele was already visited. It means it used on more places
 				//report the stacktrace of first and second usage, so that place can be found easily
 				problems.write("The element " + ele.getClass().getSimpleName()).writeln()