Unset config properties: System.getProperty should return default value or null (#10096)

Previously submitted as #10013 and reverted in #1094 

Fixes #9885

Author: zbynek

dev/core/src/com/google/gwt/dev/cfg/ConfigurationProperties.java

@@ -141,6 +141,17 @@ public List<String> getStrings(String key) {
     return properties.get(key);
   }
 
+  /**
+   * Returns all the values of a multi-valued configuration property, or null
+   * if not found.
+   *
+   * <p>A single-valued and unset configuration property will be returned as a list
+   * containing one null.
+   */
+  public List<String> getStringsOrNull(String key) {
+     return properties.get(key);
+  }
+
   /**
    * Reads a configuration property as a comma-separated list of strings.
    * It may be a single-valued or multi-valued property. If multi-valued,

dev/core/src/com/google/gwt/dev/jjs/impl/ResolvePermutationDependentValues.java

@@ -47,6 +47,7 @@
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Set;
 
 /**
@@ -74,21 +75,28 @@ public void endVisit(JPermutationDependentValue x, Context ctx) {
     }
 
     private JExpression propertyValueExpression(JPermutationDependentValue x) {
-      List<String> propertyValues = props.getConfigurationProperties().getStrings(x.getRequestedValue());
-
-      String propertyValue = propertyValues.isEmpty() ? null : Joiner.on(",").join(propertyValues);
-
-      if (propertyValue != null) {
+      List<String> propertyValues = props.getConfigurationProperties()
+          .getStringsOrNull(x.getRequestedValue());
+      if (propertyValues != null) {
         // It is a configuration property.
-        return program.getLiteral(x.getSourceInfo(), propertyValue);
+        // If no values are set, propertyValues is either empty (multivalued properties)
+        // or contains a single null (other properties).
+        if (propertyValues.stream().anyMatch(Objects::nonNull)) {
+          return program.getLiteral(x.getSourceInfo(),
+              Joiner.on(",").skipNulls().join(propertyValues));
+        }
+        if (x.getDefaultValueExpression() != null) {
+          return x.getDefaultValueExpression();
+        }
+        return program.getLiteralNull();
       }
 
       if (isSoftPermutationProperty(x.getRequestedValue())) {
         JMethod method = getOrCreateSoftPropertyMethod(x.getSourceInfo(), x.getRequestedValue());
         return new JMethodCall(x.getSourceInfo(), null, method);
       }
 
-      propertyValue = commonPropertyAndBindingInfo.getPropertyValue(x.getRequestedValue());
+      String propertyValue = commonPropertyAndBindingInfo.getPropertyValue(x.getRequestedValue());
 
       if (propertyValue != null) {
         return program.getLiteral(x.getSourceInfo(), propertyValue);

user/test/com/google/gwt/dev/jjs/SystemGetPropertyTest.gwt.xml

@@ -33,4 +33,6 @@
   <set-property name="someOtherDynamicProperty" value="blue">
     <when-property-is name="collapsedProperty" value="two"/>
   </set-property>
+  <define-configuration-property name="configPropertyUnset" is-multi-valued="false"/>
+  <define-configuration-property name="multivaluedConfigPropertyUnset" is-multi-valued="true"/>
 </module>

user/test/com/google/gwt/dev/jjs/test/SystemGetPropertyTest.java

@@ -36,5 +36,7 @@ public void testBindingProperties() {
     String expectedResult = "safari".equals(System.getProperty("user.agent")) ?
         "InSafari" : "NotInSafari";
     assertEquals(expectedResult, System.getProperty("someDynamicProperty"));
+    assertEquals("foo", System.getProperty("configPropertyUnset", "foo"));
+    assertNull(System.getProperty("configPropertyUnset"));
   }
 }