TEST(StringBuilderTest, Substring)
{
    { // Default constructed.
        StringBuilder builder;
        String substring = builder.substring(0, 10);
        EXPECT_EQ(emptyString(), substring);
    }
    { // With capacity.
        StringBuilder builder;
        builder.reserveCapacity(64);
        builder.append("abc");
        String substring = builder.substring(2, 10);
        EXPECT_EQ(String("c"), substring);
    }
}