type migration: allow add root if type parameters are already exist (example: IDEA-150172)

java/typeMigration/testData/inspections/guava/fluentIterableElementTypeChanged.java

@@ -0,0 +1,10 @@
+import com.google.common.base.Function;
+import com.google.common.collect.FluentIterable;
+
+import java.util.List;
+
+public class FluentTransformer {
+  public <U, V> Fluen<caret>tIterable<V> transformGeneric(FluentIterable<U> p1, Function<U, V> p2) {
+    return p1.transform(p2);
+  }
+}

java/typeMigration/testData/inspections/guava/fluentIterableElementTypeChanged_after.java

@@ -0,0 +1,9 @@
+import com.google.common.base.Function;
+
+import java.util.stream.Stream;
+
+public class FluentTransformer {
+  public <U, V> Stream<V> transformGeneric(Stream<U> p1, Function<U, V> p2) {
+    return p1.map(p2::apply);
+  }
+}