Make [toString] for union types more user friendly.

editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/type/UnionTypeImpl.java

Index: editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/type/UnionTypeImpl.java
diff --git a/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/type/UnionTypeImpl.java b/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/type/UnionTypeImpl.java
index 5970002acf70200514b0baf6099d986360eb75d4..ea282897bb1124ade7a86c22162f3de0b26d989f 100644
--- a/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/type/UnionTypeImpl.java
+++ b/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/type/UnionTypeImpl.java
@@ -94,13 +94,13 @@ public class UnionTypeImpl extends TypeImpl implements UnionType {
   @Override
   public String getDisplayName() {
     StringBuilder builder = new StringBuilder();
-    String prefix = "{";
+    String prefix = "(";
     for (Type t : types) {
       builder.append(prefix);
       builder.append(t.getDisplayName());
-      prefix = ",";
+      prefix = " or ";
     }
-    builder.append("}");
+    builder.append(")"); // Suffix.
     return builder.toString();
   }
 
@@ -126,13 +126,13 @@ public class UnionTypeImpl extends TypeImpl implements UnionType {
 
   @Override
   protected void appendTo(StringBuilder builder) {
-    String prefix = "{";
+    String prefix = "(";
     for (Type t : types) {
       builder.append(prefix);
       ((TypeImpl) t).appendTo(builder);
-      prefix = ",";
+      prefix = " or ";
     }
-    builder.append("}");
+    builder.append(")");
   }
 
   @Override