Remove use of spring-core's Assert from PropertiesLauncher

Fixes gh-23474

Author: wilkinsona

spring-boot-project/spring-boot-tools/spring-boot-loader/pom.xml

@@ -19,12 +19,7 @@
 		<developerConnection>${git.developerConnection}</developerConnection>
 	</scm>
 	<dependencies>
-		<!-- Optional (must never have compile/runtime time dependencies) -->
-		<dependency>
-			<groupId>org.springframework</groupId>
-			<artifactId>spring-core</artifactId>
-			<optional>true</optional>
-		</dependency>
+		<!-- Mmust never have compile/runtime time dependencies -->
 		<!-- Test -->
 		<dependency>
 			<groupId>org.springframework.boot</groupId>

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/PropertiesLauncher.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -43,7 +43,6 @@
 import org.springframework.boot.loader.archive.ExplodedArchive;
 import org.springframework.boot.loader.archive.JarFileArchive;
 import org.springframework.boot.loader.util.SystemPropertyUtils;
-import org.springframework.util.Assert;
 
 /**
  * {@link Launcher} for archives with user-configured classpath and main class via a
@@ -370,7 +369,9 @@ private ClassLoader wrapWithCustomClassLoader(ClassLoader parent, String classNa
 		if (classLoader == null) {
 			classLoader = newClassLoader(type, NO_PARAMS);
 		}
-		Assert.notNull(classLoader, "Unable to create class loader for " + className);
+		if (classLoader == null) {
+			throw new IllegalArgumentException("Unable to create class loader for " + className);
+		}
 		return classLoader;
 	}