IDEA-151950 Decompiler doesn't work for classes from JDK 9 - support java 9 string concatenation

testData/src/pkg/TestStringConcat.java

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2000-2014 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package pkg;
+
+public class TestStringConcat {
+  public String test1(String prefix, int a) {
+    return prefix + a;
+  }
+
+  public String test2(String var, int b, Object c) {
+    return "(" + var + "-" + b + "---" + c + ")";
+  }
+}