fix(material/button-toggle): incorrect event source in some cases (#25544)

Removes some logic that was trying to incorrectly match the button toggle being clicked with the model value. The logic wasn't necessary since we already know which toggle the user interacted with.

Fixes #25489.

(cherry picked from commit 335aa0a)

Author: crisbeto

src/material/button-toggle/button-toggle.ts

@@ -264,10 +264,8 @@ export class MatButtonToggleGroup implements ControlValueAccessor, OnInit, After
   }
 
   /** Dispatch change event with current selection and group value. */
-  _emitChangeEvent(): void {
-    const selected = this.selected;
-    const source = Array.isArray(selected) ? selected[selected.length - 1] : selected;
-    const event = new MatButtonToggleChange(source!, this.value);
+  _emitChangeEvent(toggle: MatButtonToggle): void {
+    const event = new MatButtonToggleChange(toggle, this.value);
     this._controlValueAccessorChangeFn(event.value);
     this.change.emit(event);
   }
@@ -305,9 +303,9 @@ export class MatButtonToggleGroup implements ControlValueAccessor, OnInit, After
     // the side-effect is that we may end up updating the model value out of sequence in others
     // The `deferEvents` flag allows us to decide whether to do it on a case-by-case basis.
     if (deferEvents) {
-      Promise.resolve().then(() => this._updateModelValue(isUserInput));
+      Promise.resolve().then(() => this._updateModelValue(toggle, isUserInput));
     } else {
-      this._updateModelValue(isUserInput);
+      this._updateModelValue(toggle, isUserInput);
     }
   }
 
@@ -369,10 +367,10 @@ export class MatButtonToggleGroup implements ControlValueAccessor, OnInit, After
   }
 
   /** Syncs up the group's value with the model and emits the change event. */
-  private _updateModelValue(isUserInput: boolean) {
+  private _updateModelValue(toggle: MatButtonToggle, isUserInput: boolean) {
     // Only emit the change event for user input.
     if (isUserInput) {
-      this._emitChangeEvent();
+      this._emitChangeEvent(toggle);
     }
 
     // Note: we emit this one no matter whether it was a user interaction, because

tools/public_api_guard/material/button-toggle.md

@@ -92,7 +92,7 @@ export class MatButtonToggleGroup implements ControlValueAccessor, OnInit, After
     _controlValueAccessorChangeFn: (value: any) => void;
     get disabled(): boolean;
     set disabled(value: BooleanInput);
-    _emitChangeEvent(): void;
+    _emitChangeEvent(toggle: MatButtonToggle): void;
     _isPrechecked(toggle: MatButtonToggle): boolean;
     _isSelected(toggle: MatButtonToggle): boolean;
     get multiple(): boolean;