Some string and minor optimizations

core/compiler.common/src/org/jetbrains/kotlin/name/ClassId.java

@@ -105,8 +105,16 @@ public final class ClassId {
 
     @NotNull
     public static ClassId fromString(@NotNull String string, boolean isLocal) {
-        String packageName = StringsKt.substringBeforeLast(string, '/', "").replace('/', '.');
-        String className = StringsKt.substringAfterLast(string, '/', string);
+        int lastSlashIndex = string.lastIndexOf("/");
+        String packageName;
+        String className;
+        if (lastSlashIndex == -1) {
+            packageName = "";
+            className = string;
+        } else {
+            packageName = string.substring(0, lastSlashIndex).replace('/', '.');
+            className = string.substring(lastSlashIndex + 1);
+        }
         return new ClassId(new FqName(packageName), new FqName(className), isLocal);
     }
 

core/compiler.common/src/org/jetbrains/kotlin/name/NameUtils.kt

@@ -42,7 +42,7 @@ object NameUtils {
             // NB `uppercase` uses Locale.ROOT and is locale-independent.
             // See Javadoc on java.lang.String.toUpperCase() for more details.
             if (Character.isJavaIdentifierStart(str[0]))
-                str.substring(0, 1).uppercase() + str.substring(1)
+                str[0].uppercase() + str.substring(1)
             else
                 "_$str"