Use StringBuilder instead of StringBuffer

tool/src/org/antlr/v4/tool/Grammar.java

@@ -385,7 +385,7 @@ public class Grammar implements AttributeResolver {
         List<Grammar> grammarsFromRootToMe = getOutermostGrammar().getGrammarAncestors();
         String qualifiedName = name;
         if ( grammarsFromRootToMe!=null ) {
-            StringBuffer buf = new StringBuffer();
+            StringBuilder buf = new StringBuilder();
             for (Grammar g : grammarsFromRootToMe) {
                 buf.append(g.name);
                 buf.append('_');

tool/test/org/antlr/v4/test/BaseTest.java

@@ -775,7 +775,7 @@ public abstract class BaseTest {
 	}
 
 	public static class StreamVacuum implements Runnable {
-		StringBuffer buf = new StringBuffer();
+		StringBuilder buf = new StringBuilder();
 		BufferedReader in;
 		Thread sucker;
 		public StreamVacuum(InputStream in) {

tool/test/org/antlr/v4/test/TestFastQueue.java

@@ -54,7 +54,7 @@ public class TestFastQueue {
         q.add("c");
         q.add("d");
         q.add("e");
-        StringBuffer buf = new StringBuffer();
+        StringBuilder buf = new StringBuilder();
         while ( q.size()>0 ) {
             String o = q.remove();
             buf.append(o);
@@ -67,7 +67,7 @@ public class TestFastQueue {
     }
 
     @Test public void testQueueThenRemoveOneByOne() throws Exception {
-        StringBuffer buf = new StringBuffer();
+        StringBuilder buf = new StringBuilder();
         FastQueue<String> q = new FastQueue<String>();
         q.add("a");
         buf.append(q.remove());